Pre-heat your oven to 200C/180C Fan/Gas Mark 6. Line an 8" Square Pan with greaseproof paper.
Combine the melted butter, bananas, granulated sugar and light brown sugar in a large mixing bowl or the bowl of a stand mixer. Combine on high speed for 3-5 minutes. Add the eggs and vanilla and combine again.
Fold in the flour, baking powder and chopped chocolate until all combined. Pour into the lined tin.
Bake 30-35 minutes, it may still be a little wobbly in the middle (but only a little)! Let cool on a wire rack before drizzling over some melted chocolate (25g chocolate of choice, melted in the microwave in 20 second intervals). Slice and serve!