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
The temperature of the ingredients is very important when making cake. You will have problems if you start from cold ingredients. I'm afraid that there are no good solutions if you don't have a warm room, all work arounds are quite laborious.
Microwaving the butter is not a good solution. It is a very uneven method of heating. You always end up with too-hard spots and melted spots beside the somewhat softened butter.
The proper way to have butter at the correct consistency is to keep it for at least overnight in an environment which is between 20 and 30 degrees Celsius, and has ambient heat. Putting it on top of a working oven and similar is problematic. If you don't have a room in your house which is at that temperature, you might consider making a water bath for your butter. Warm a Dutch oven with water in the stove until it reaches 30 Celsius, pack the butter in a smaller container, float it in the Dutch oven. Pack the whole thing in a few blankets. It should soften the butter overnight without getting too cold (you said your room doesn't fall below 10 degrees).
A workaround is to cut up the butter into very thin pieces (2-3 mm) and then put cream them with the sugar more than usual. The friction from the mixer will warm it up. This can work well if you have a stand mixer, but if you are doing it with a hand mixer, you'll need to stand there for 15-20 minutes just beating the butter. A water bath might help, but you really have to pay lots of attention not to melt the butter.
You can also consider replacing the butter. Shortening and margarine are much more beatable at low temperatures than butter. You'll get worse taste though, and some change in texture. You have to use a solid fat, not an oil. If you use a margarine, use a full-fat one, not reduced-fat spreads.
The butter is your most obvious problem, but you'll never get perfect results if your eggs are cold. The texture might be good enough for you when you bake with cold eggs; I've seen many bakers who don't mind it. But if you are really picky about your cake quality, you also need to warm the eggs overnight. Alternatively, you can quickly warm them in a water bath after breaking.
Adding milk or other liquids is counterproductive. It will change the texture of the finished cake a lot. Your recipe is a slightly modified pound cake, standard pound cake recipes don't have liquid (and neither do many other cake types). The only reason for dairy I can imagine there would be to add some acid for the baking soda to work - but then it should be buttermilk or yogurt, not milk. The more liquid you add, the less cakelike and more quickbreadlike your result will be.