First make up the brownie mix. Break up the Extra Dark Chocolate into a heatproof bowl. Add the butter and place over a saucepan of barely simmering water until melted. Remove from the water and cool for 10 minutes.
Put the egg, sugar and Vanilla Extract in a bowl and whisk until well blended and creamy in texture. Beat in the melted chocolate.
Now make the cookie dough. Line 3 large baking trays with baking parchment. In a bowl, beat together the butter and sugar until creamy and well blended. Beat in the egg yolk.
Sift the flour and Baking Powder on top and add the Chocolate Chips. Mix together to form a fairly firm dough.
Divide the mixture into 12 equal portions and roll each into a ball. Cut the balls in half and arrange these spaced well apart on the trays, pressing each half down slightly to make chunky half circle shapes. Chill until required.
When ready to cook, preheat the oven to 180˚C (160˚C fan assisted oven, 350˚F, gas mark 4). Using a teaspoon, scoop a portion of the brownie mixture and place, overlapping slightly, next to each cookie portion. Press down a little to join the 2 mixtures together then form into an overall rough round shape, but without flattening the brownie part.
Bake for about 15 minutes until risen, just firm and the cookie part is pale golden – the centre of the brownie should remain soft. Leave to cool for 5 minutes then transfer to wire racks to cool. Delicious served warm or cold.