Firstly, it strikes me as odd that your recipe has no raising agent - no baking powder, no bicarbonate/baking soda, no self-raising flour. Unless you're whipping a lot of air into the batter, the cakes will barely rise, and you will end up with 'cookies'.
I would add 2 teaspoons of baking powder and 1/4 teaspoon of bicarbonate of soda (baking soda) and see if that helps.
As for the temperature of your oven, I wouldn't go so far as to halve the temperature and double the time. The idea is that you need enough heat for the cakes to rise at a decent rate then form a crust on the outside. 100 degrees will do little but slowly dry the cakes out.
Domestic ovens are rarely well calibrated, so your oven may be running hotter than the dial indicates. Invest in an oven thermometer to make sure you are setting the temperature correctly: I have had an oven run 40 degrees hotter than the dial said before now!
If you haven't got the time to get a thermometer, try setting the oven about 20 degrees cooler on the dial, to around 180 degrees.
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
Adjusting a basic cake (either from a boxed mix or from scratch) is easy if the cake has no or very little flavour of its own.
Using a boxed mix might be more difficult because they often have a generous amount of flavouring, typically vanilla, even if it does not explicitly say so on the box front. Check the ingredients list or use a mix you know - a little vanilla is ok.
You can either prepare the basic recipe, then divide the batter (I'll call this method 1 for now) or divide the ingredients and make separate batches (method 2). The latter might be difficult if you need to split eggs, but it's possible. See this question.
Now for the adjustments you asked for, amounts suggested should be enough for 1/3 to 1/2 of normal recipe:
Assuming you want real vanilla, split 1/2 vanilla bean and scrape out the seeds. For method 1, add seeds to finished batter (to avoid lumps, mix with a few pinches of sugar until "sprinklable") or stir into liquid according to recipe for method 2.
Add 1 to 1 1/2 tbsp of cocoa powder (the real stuff, not drink mix!) either to the dry ingredients or flour (method 2) or sive over the batter and stir (method 1). You might have to add a tsp of extra liquid - or not. Hard to say w/o knowing the original recipe. Do not use normal chocolate, because this may ruin your batter, especially if you started from a sponge-type, pound cake is a bit more forgiving. You can make chocolate cake with molten chocolate, but this needs more adjustments / a different recipe.
A strawberry taste can only be done with artificial flavouring. I have never used this, so I won't give a recommendation here. Read the label.
Replace ca. 1 tbsp of liquid with liquour of your choice (method 2) or add 1/2 tbsp at the end (method 1). Poking the baked warm cupcakes a few times with a skewer and drizzling with 1/2 to 1 tsp of alcohol (or alcohol with sugar syrup) might be considered method 3.
Don't forget the powers of your spice cupboard: Cinnamon, nutmeg, cardamom, even ground cloves, give a lot of flavour w/o the danger of throwing tipping the dry-to-liquid balance of your recipe. A very small pinch (cloves, be careful with these!) to a shake or two (cinnamon) is all you need.
Note that you can mix-and match the methods above, e.g. add one tbsp cocoa and 1 tsp of brandy or dissolve the vanilla bean scrapings in a bit of of whiskey. Chocolate goes nicely with a pinch of cinnamon, you could even go all "Christmasy" (ha, and to post this on Easter...) with gingerbread spice or brandy, cinnamon & nutmeg for "eggnogg".