1. Cream together softened butter and sugar until light and fluffy, about 2–3 minutes.

2. Mix in the egg yolk and vanilla extract until combined.

3. In a separate bowl, whisk together flour, cocoa powder, and salt.

4. Gradually add dry ingredients to the wet, mixing until a soft dough forms.

5. Wrap dough in plastic wrap and chill for at least 30 minutes.

6. Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.

7. Scoop and roll dough into 1-inch balls, placing them on the sheet spaced apart.

8. Use your thumb or back of a spoon to gently press a shallow well in each ball.

9. Bake for 10–12 minutes until set but still soft.

10. Cool cookies completely on a wire rack.

11. Heat heavy cream until steaming, then pour over chocolate chips.

12. Let sit 1 minute, then stir into smooth ganache.

13. Spoon or pipe ganache into each cooled cookie center.

14. Top with nuts, sprinkles, or powdered sugar if desired.

15. Let cookies set at room temp or chill briefly before serving or storing.