Pre heat oven to 180°C fan forced.

In a saucepan over low heat, melt the butter, dark chocolate, milk chocolate and sugar, stirring constantly.

Allow to cool slightly and pour into a large bowl.

Once mixture has cooled to about room temperature add the eggs and vanilla and mix well.

Sift in the flour and cocoa, add the white chocolate and stir gently until just combined.

Pour into a 20cm square cake tin lined with baking paper and bake for 20 minutes until just set.

Leave to cool in the tin, then remove from tin, wrap in plastic wrap and place in fridge overnight.

Cut into 16 squares and store in an airtight container.