In the bowl of the food processor, combine the sugar and butter. Incorporate the egg, egg yolk and vanilla. Gradually add the dry ingredients and mix everything together.
Finish by adding the Smarties (keep a little for decoration) and the chocolate chips. Mix well.
Chill for 10 minutes, then form small balls of dough in your hand, place them on a baking sheet covered with baking paper, spacing them 5cm apart. Place the remaining Smarties to decorate the top of each ball.
Bake for about 15 minutes until the cookies are lightly golden. Let cool on a rack, they will harden slightly while keeping their soft core.