experiment
4 eggs (brittish eggs are puny and stupidly small, if you have access to proper eggs, 3 should do)
250g plain flour
100g sugar
100g chunky peanut butter
2 tsp baking powder
1 tsp baking soda
1/2 tsp salt
2 tbsp cocoa powder

1 - Prep all the ingredients so you can easily add them to the mix one after another (I put them in cups/glasses (baking powder, soda and salt in the same one, because a single container for each is a waste))
2 - crack all 4 eggs and mix them thorhoughly with the electric mixer
3 - add the sugar, baking powder+soda+salt  and peanut butter to the eggs, making sure they are thoroughly mixed before adding the next one. Do not stop the electric mixer throughout the entire process.
4 - put the flour and the cocoa powder on a strainer and sieve it over the egg mix
5 - fold the batter until it is completely mixed and there are no pockets of dry flour
6 - 30 mins in the oven @ 180c (356 freedom degrees)

Remember that this recipe made an extremely sticky batter, if yours also turns out like that, add some low-fat youghurt or milk to get it to a better consistency