1. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper

2. In a bowl whisk together flour, cocoa powder, baking soda, and salt

3. In a mixer beat softened butter with granulated and brown sugars until light and fluffy

4. Add egg, egg yolk, vanilla extract, and peppermint extract, mixing until smooth

5. Gradually incorporate dry ingredients into wet mixture and fold in white chocolate chips and peppermint chocolate chips

6. Scoop 1 1/2 tablespoon portions of dough onto prepared sheets, spacing 2 inches apart

7. Bake cookies for 9–11 minutes until edges are set and centers are slightly soft

8. Let cookies rest on baking sheet for 5 minutes, then transfer to a wire rack to cool completely

9. If desired, drizzle cooled cookies with melted white chocolate and sprinkle crushed peppermint on top