Preheat the oven to 180 degrees/gas mark 4. Line two large baking sheets with non stick paper.

 

Beat the butter and sugar together in a large bowl until light and fluffy.

 

In a separate bowl mix the egg and vanilla extract together, then gradually add to the butter mixture until smooth. Mix in the flour, cocoa powder, bicarbonate of soda and salt until well combined. Add 100g each of the milk and white chocolate, then mix well.

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Spoon 6 heaped tablespoons of the mixture onto each baking sheet, spacing them well apart. Divide the rest of the chocolate among the cookies and press in lightly.

 

Bake in a preheated oven for 15-17 minutes. Leave to cool on the baking sheet for 5 minutes then move them to a wire rack to cool some more.