1. In a large mixing bowl, beat butter, granulated sugar, and brown sugar until light and fluffy.

2. Add in egg yolk and vanilla extract; mix until smooth.

3. In a separate bowl, whisk together flour, cocoa powder, and salt. Gradually add dry ingredients to the butter mixture and mix just until combined.

4. Wrap the dough in plastic and chill for 30 minutes to firm up.

5. Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.

6. Roll dough into 1-inch balls. Roll each ball in granulated sugar and place on baking sheet.

7. Use your thumb or the back of a spoon to gently press an indentation into the center of each cookie.

8. Bake for 10–12 minutes. If needed, press centers again after baking.

9. While cookies bake, heat cream until just simmering and pour over chocolate in a bowl. Let sit for 2 minutes, then stir until smooth ganache forms.

10. Spoon or pipe ganache into the center of each slightly cooled cookie. Let set before serving.