Preheat the oven to 350°F.

1. In a robot, mix together butter and powdered sugar. Add the egg.

2. Add flour mixed with cocoa powder to obtain a soft dough.

3. Shape into rolls and wrap in plastic wrap. Refrigerate for 1 hour.

4. Cut the dough roll into equal slices.

5. Place the slices on a parchment-lined baking sheet and bake for 10 to 12 minutes. Let cool on a rack.

6. Decorate the edges of the cookies with small rainbow candies. To do this, dip into melted chocolate and then roll in the candies.