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
It depends a lot on what you expect. I wouldn't call it a mess. The result will be edible, and will have a similar flavor to a brownie. The texture will be very different from a brownie. It will be dry and dense. Personally, I wouldn't eat it.
First, the fat plays important roles in baked goods besides taste. I am surprised to see a recipe which uses melted fat (so no creaming) and no baking powder. This thing (the original one) is going to be denser than chewy cookies - maybe OK for you. If you reduce the fat from the original recipe, you will get a very dry result. An airy cake which is dry is not too good, but can be eaten when combined with e.g. yogurt or other wet things. Something which is both dense and dry will taste like chicken food mix run through a dehydrator. If you have a recipe for baking, don't reduce the fat. Also, never use spreads or margarines or anything else which is not real fat. It does not act like real fat, so it doesn't work the way it should. It is likely to contain lots of water and gums, and melt into them when baked. For baking, always use butter, lard or shortening. (It is OK to use vegetable oils for dense recipes which direct you to melt the butter). You will need your full share of fat. Some people replace part of the butter with applesauce, but you can't replace all of it. Also, I don't know how it will act in a non-leavened recipe.
Second, you need the sugar the same way you need the fat. It retains moisture and makes the brownies soft. Honey is already a risky proposition, but partial replacement will work. But do not reduce the sugar by half, it will make dry, unpleasant brownies. Also, I don't know what "no calorie sugar" is, but it sounds like some artificial sweetener dispersed in a filler. I don't know what the filler is made from, but it may not be good for baking.
You can't always replace wheat flour with other flours, but finelly milled oats should work for brownies and cakes.
Using a vegetable or fruit puree in place of the liquid is normally a good substitution, but your recipe doesn't have liquid. Just adding cabbage will destroy the liquid balance. You could theoretically replace part of the eggs with an equal amount of cabbage puree by weight, but because eggs also have important roles in baking (they lubricate and set the dough), you shouldn't nomrally replace them.
So, the replacement will bake into something you can bite and chew and swallow, but I wouldn't call it brownie. It also wouldn't be a tasty non-brownie, at least not in my eyes. Also, if you go ahead and make the stuff, you don't even win much in calories. Your original recipe is about 2950 kcal, and you get 820 g of brownies, resulting in 359 kcal/100 g. Your replacement recipe is 1610 kcal but only 530 g of brownies, which makes it 302 kcal/100 g (still a very caloriedense food) - and that if you use these substitution products, which are bad for baking. So, you only lose 15% of the calories overall, but you get something which tastes nowhere near the original. It will have cocoa flavor, but it won't be moist and creamy. I would rather eat 100 g of real brownies than 115 g of brownies made with your recipe.