How to adjust Baker's Percentage when Adding Ingredients
Cocoa powder is a drying agent. So you would want to remove a portion of the flour, and replace with a smaller portion of the cocoa powder plus an extra pinch of baking soda to counteract the acid nature of cocoa powder.

So if you were to follow the advice of 480g flour and 120g cocoa powder, (which sounds good) I would add a 1/4tsp of baking soda in addition to the baking powder.

I would try that out, keeping in mind that cocoa in addition to being acidic acts as a drying agent and so you may find that the above suggestion yields a slightly dryer donut than the original recipe. Adjust incrementally by tbsp of milk.

As a side note, we've all had fried chocolate donuts before and they probably haven't been bitter or burnt tasting. If you use way too much cocoa, sure. Creating recipes can be tricky, keep in mind it often takes some experimentation to perfect.