Chocolate Brownie Mug Cake

Place the butter in a 1 cup-sized cup or mug, and microwave for 10-20 seconds until melted. Add both sugars and cocoa powder and beat using a fork until combined. Add the egg yolk, vanilla, flour, baking powder and salt and beat again until thoroughly combined. Stir in half of the white and dark chocolate chips. Cook in the microwave for 30 seconds, then scatter the remaining chocolate chips over the top and return it to the microwave for a further 30 seconds. Remove from the microwave and allow to cool a little before serving.