All recipes are chemical equations ..
1. that much cocoa is supplying structure as well as taste to the completed product. Cornstarch will only provide the non-gluten bulk ..
2. natural cocoa is acidic in nature: to balance this, there is usually an alkaline item in the recipe: bicarb soda, buttermilk, sour cream, yoghurt etc. The chemical reaction neutralizes but it also acts as a secondary leavener. Removing the cocoa, means that another item will have to be introduced to counter the alkaline ingredient
3. when you remove one of the catalsyts of the acid/alkali reaction, you will have to add some type of leavener or the item may not rise as it is supoosed to ..
4. cocoa in a baked product is very drying, especially that amount. More liquid is added to balance this tendency. Removing the cocoa will unbalance the moisture content
Suggestion: look for another recipe ..