Preheat the oven to 180ºC. Chop the white and milk chocolate into chunks.
Start with melting the butter and dark chocolate in a bowl. Either use the traditional method over a pan of simmering water or melt in the microwave. Once melted, stir and leave to cool down.
Mix the eggs with the sugar with an electric mixer for 5minutes or until the mixture doubled in size and becomes pale.
Now pour the cooled chocolate mixture into the egg mixture and start folding them together with a spatula. Do this very gentle and slow. Next comes the flour and cocoa. Add them by utilising a sieve and use a spoon to push the flour and cocoa powder through the sieve. Gently fold the flour and cocoa powder under the mixture. This will take some time and do not rush this step. Finally stir the white and milk chocolate chunks into the mixture.
Line a suitable kitchen tin with parchment (20cm x 20cm) and pour the chocolate mixture into the tin. Bake for 25min or until the brownie doesn’t wobble in the middle when you shake the tin. Take out of the oven and leave to cool before taking it out of the tin and cutting it into desired forms e.g. squares or triangles.