Add the caster sugar and sifted cocoa powder to the melted butter and stir.
Add the vanilla extract and the eggs, and whisk to combine.
Sift the plain flour into the bowl, and gently fold this through the mixture.
Add half of the white chocolate and dark chocolate chunks, reserving the rest to go on top of the brownie. Fold gently through the batter.
Pour the brownie mixture into the prepared tin and sprinkle the remaining white and dark chocolate chunks across the top. Bake for 30-35 minutes.