Preheat your oven to 160 degrees and line the base and sides of a 20cm square baking tin with baking paper – this will help you to remove the brownies once they are cooked.
In a large saucepan (you want it to be big enough to add the extra ingredients to later), melt the butter, cocoa and sugar over a low heat.
Remove the saucepan from the heat and add the vanilla essence and egg and whisk the mixture as you go.
Sift the flour into the saucepan and whisk to combine.
Add approximately 150g of the white chocolate chips and all of the milk chocolate chips and stir to combine.
Pour the brownie mixture into the prepared tin and smooth the surface. Sprinkle the remaining white chocolate chips over the top.
Place the brownies into the oven to bake for 35 minutes.
Carefully remove the cooked brownies from the oven and allow them to cool in the tin before removing, cut into small pieces .