Preheat oven to 160. Paper line a 20 x 30 cm sponge roll tin.
Melt the first measure of chocolate.
Beat sugar, eggs and vanilla in food processor or with a hand held mixer. Add melted chocolate, flours and cocoa.
Stir in the extra chopped chocolate, then pour into a prepared tin.
Bake for 50 mins until firm.
Cool in the tin and cut into small bars or chunks. When cold, store in an airtight container. Dust with icing sugar to serve.
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