Preheat oven to 350(180C). Melt butter in a medium sized saucepan over a low heat. Once butter is completely melted mix in cocoa.
Remove from heat and stir in sugar. Add the eggs and beat well using a wooden spoon. Sift flour and baking powder into the mixture. Add the vanilla essence and mix till combined.
Line a 11” x 7” baking pan or simular with parchment paper. Pour half of the mixture in making sure to cover the base. Sprinkle the chopped chocolate evenly on top then pour other half of mixture over. You may need to use a spatula to spread.
Making three lines lengthwise put small blobs of peanut butter on top. Using a knife swirl through. Bake for 25-30 minutes. Allow to cool before slicing.