Pre-heat oven to 200°c (180°c for a fan assisted oven or Gas Mark 6). Grease and line your tin (20cm square is ideal) with grease-proof paper.
Melt the dark chocolate and butter/margarine in a bowl over a bain-marie or in short bursts in the microwave. Out aside to cool slightly.
In a large bowl, whisk the eggs and sugar on a medium speed until thick and creamy.
Pour the chocolate mixture into the egg mixture and fold together.
Sift in the flour and cocoa flour, Roughly chop the milk chocolate and white chocolate and add to the mixture. Gently fold all together.
Pour the mixture into the pre-lined tray, ensuring it is evenly distributed, right to the edges.
Bake for 25-30 minutes until the top is crusty.
Leave to cool completely before cutting.