There are several aspects of the chemistry of the chocolate cake being converted to a white cake that are not covered in the existing answers.
Fat content of cocoa powder
You cannot simply replace the cocoa powder with flour, because it compared to flour, cocoa powder:
- Contains more fat (about 10-12 percent by weight, for grocery store brands; more for some premium brands up to about 25%)
- Is more acidic
- Is far, far more flavorful (okay, that part is not desired in a white cake)
You should use slightly less flour than there was cocoa, to avoid adding flour by weight equivalent to the fat in the original recipe which would throw the balance off. I would suggest a half cup plus 2 tbls of flour instead of 3/4 cup of cocoa as a starting point.
Flavor balance
With the astringency of the cocoa removed from the recipe, the flavor balance may be too sweet. You might want to try reducing the sugar by about 1/4 cup - 1/2 cup.
Acid and Leavening
The original recipe contains both baking soda (which requires an additional acid from another ingredient) and baking powder as leavening.
When you remove the cocoa, you are removing the acid that the baking soda was intending to react with.
Therefore, you need to remove the baking soda, replacing it with baking powder which brings its own acid. You might need a slightly larger amount, as part of the weight of baking powder is the acid, so there is less leaving per unit than with baking soda. I would suggest an additional 1/2 teaspoon as a starting point.
Boiling water
The boiling water is specifically intended, I infer from experience with many chocolate cake recipes, to bloom the flavor in the cocoa. When you remove the cocoa from the recipe, you remove the need to use boiling water to enhance the chocolate flavor.
Furthermore, the reason water is used rather than milk or another liquid is to interfere less with the perception of chocolate flavor. Therefore, you may choose to use another liquid like milk or additional coconut milk which will enhance the flavor and tenderness of your final cake.
Vegetable Oil
Vegetable oil is chosen in this recipe to avoid interfering with the flavor from the cocoa.
You may continue to use it, or you may find that butter or even coconut oil give a more agreeable flavor for the final product you are creating.
Method
With all of the above changes, there will no doubt be some changes to the method. Since you have only listed the ingredients, and not the actual steps, it is harder to be specific here.
However, what will be left is a fairly standard sponge cake, so should modify the steps to the standard creaming method:
- Sift the dry ingredients together; combine the wet ingredients other than butter and sugar
- Cream the butter and sugar(s) together
- Add in the eggs one at a time, beating until smooth.
- Add the dry mix alternating two or three times with the wet mix, beating each time for 30 seconds or so
- Beat one or two minutes longer to develop structure
Flavorings
For coconut, using coconut oil in lieu of vegetable, and more coconut milk in lieu of water, and adding coconut extract should do the trick. The recipe should also be able to carry a small amount of shredded coconut if you choose, although that will change the texture.
For vanilla, try butter and milk instead of oil and water. Add vanilla extract.
For lemon, start as with vanilla, and add lemon zest to the batter (and maybe lemon extract), and possibly, should you choose, a touch of yellow food color. I would not add lemon juice as it has a profound effect on the chemistry; instead I would make a nice lemon icing or glaze that has the tartness.
Conclusion
Despite all of the above, I would not try to convert a recipe that is specifically designed for chocolate to a white cake; too many aspects of the cake will change. Instead, I suggest finding a truly outstanding white or yellow cake recipe to use.
Best Answer
Why the doming happens
When you heat leavened dough, two things happen:
The pattern you are seeing means that your cake gets hotter on the sides than in the middle. The sides get set early and stop rising, while the middle is still soft and continues rising.
This happens because the walls of the pan conduct heat to the sides of the cake quicker than heat is conducted inside the cake. You see the phenomenon in an exaggarated form in a muffin, which is usually higher than wide: it is always rounded on top, and often split, because the liquid core from the middle continues to rise after a crust has formed on the surface.
How to prevent it
Slower heating
One good solution could involve a slower, more even heating method.
This can be accomplished by using a pan which insulates well. With an insulating pan, the sides will stay liquid longer and rise more. Unfortunately there are no insulated drop-out-bottom pans.
Porcelain
I have had good results in a porcelain or Pyrex pan. The pan insulates the cake from the bottom and the sides, but not from the top, so you will need to bake it on a lower rack and/or reduce temperature on the upper heater in order to not burn the upper surface.
Getting a cake layer out of the pan in one piece can be hard.
To deal with this problem, if your batter is not too sticky, the bottom of the pan can be lined with parchment paper. The sides can be lined with a strip of parchment or well lubricated with fat. This approach is a hassle.
Insulate a metal pan
Perhaps, when combined with other approaches, a metal cake pan with thicker walls will work well enough.
You could also try a more DIY solution, like attaching 2-3 layers of a cut-to-fit silicone rolling mat stripes to the outside of the walls of a metal pan. However, it is quite hard to come up with a good attachment method (glue could release toxic fumes at oven temperatures, if it holds at all). So this method should work, but is somewhat hard to apply.
Glass and silicone pans
There are pans with glass bottoms and silicone walls but I think these would not help because the glass would insulate better than the thin silicone.
Preheating
Perhaps preheating the detached bottom of a metal pan with the oven could help, but I haven't tried it.
Minimize the center
A second approach is to minimize the liquid core by making a thin cake.
If the middle of the cake is heated quickly enough from above and below, it will set shortly after the walls do, and there will be no unset center to "bloom".
Use a bigger pan.
I think that Americans mostly use 9 inch pans, but if you used an original sacher recipe from Austria, it is probably meant for a 26 or a 28 cm pan.
Bake the two layers separately
Divide the batter into two pans instead of making a single layer and cutting it. This is unorthodox, as you get more crust but it's better than a hunchbacked sacher. You should use a scale or at least a measuring jug to divide the batter, or you'll end up with different thicknesses. Also remember to reduce the baking time, as the core will heat up faster.
Use a toothpick for probing doneness.
Reduce the leavening
The third idea is to reduce the amount of leavening agent.
The walls will always set before the core and, if there is enough baking powder, the core will still expand a little bit more. If the concentration of baking powder is low you'll get less bubbles, so less lift.
Of course, using too little baking powder will also ruin the cake, so you'll have to be cautious and experiment a bit before hitting the correct amount.
Lower heat
The fourth approach is to give the outer portions of the cake more time to rise by baking at lower heat.
This is somewhat risky, as it can result in a different texture of the final product, due to the different rate at which water will evaporate from the dough and a longer baking time. Also, if your temperature is too low, you won't get a golden crust on a light-coloured dough. This should not be a problem for a frosted cake like the sacher.
Less gluten
The fifth approach is to use a flour with less gluten.
With less gluten you get a mesh which is less dense and needs more time until it gets firm enough to prevent rising. Use cake flour instead of all purpose flour. A bit more fat will also help to inhibit gluten development but too much will change the taste and texture.
All methods I described should attribute to a solution, but probably none of them will be sufficient by itself. You'll have to pick a combination of them and see what works best for you.
General Cake Techniques
You should also apply all the usual methods for getting a good cake:
These techniques ensure a better batter texture, which means a more even heating. They will also ensure a more consistent leavening process because:
Not following these techniques is more likely to result in a lopsided cake or a big bubble. I realize that this hasn't happened in the case you describe, but it would be too bad to get an asymmetrically risen cake after you took all the precautions against a disproportionally rising center.