Preheat the oven to 190 C / Gas 5. Line 2 to 3 baking trays with baking parchment.
Put the butter and sugar into a bowl and mix well with a wooden spoon, then beat in the egg yolk and vanilla extract. Sift together the flour, cocoa powder and a pinch of salt into the mixture, add both kinds of chocolate chips and stir until thoroughly combined.
Make 12 balls of the mixture, put them on to the prepared baking sheets, spaced well apart, and flatten slightly. Press the pieces of dark chocolate into the cookies.
Bake for 12 to 15 minutes. Leave to cool on the baking trays for 5 to 10 minutes, then using a palette knife, carefully transfer to wire rack to cool completely.
Use marshmallows and sweets etc. instead of milk chocolate and white chocolate.