1. Preheat oven to 350°F (177°C) and line baking sheets with parchment paper.

2. In a large bowl, beat softened butter with granulated and brown sugars until pale and fluffy.

3. Mix in the egg yolk and vanilla extract until just combined.

4. In another bowl, whisk flour, cocoa powder, and salt; gradually add to the butter mixture until a soft dough forms.

5. Wrap dough and chill for at least 30 minutes.

6. Roll dough into 1-inch balls and place 2 inches apart on prepared sheets.

7. Use a floured thumb or back of a teaspoon to press an indentation into each ball.

8. Bake for 10–12 minutes, rotating sheets halfway, until cookies are slightly puffed.

9. If indentations puff up, gently press them again after removing from oven.

10. Melt chocolate chips until smooth and spoon into each indentation.

11. While centers are soft, sprinkle festive decorations on top.

12. Cool completely on a wire rack before storing in airtight containers.