1 Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6 and grease a baking tray.
3 Melt the 70% cocoa chocolate in a bain marie. Remove from the heat and stir in the butter until creamy.
5 Mix together 100g sugar with the flour and the cocoa. Sieve into a bowl.
7 Chop the white chocolate and the dark chocolate.
9 Beat the eggs and the remaining sugar until foamy, white and creamy.
11 Carefully stir in the cooled chocolate-butter mixture. Add the sieved ingredients and mix well.
13 Fold in the chopped chocolate and transfer the mixture to the prepared baking tray. Bake in the middle of the oven for around 30 min.
14 Leave to cool in the baking tray, then cut into squares and decorated with the chocolate glaze if desired.