1. In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.

2. In a large mixing bowl, cream the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2–3 minutes.

3. Add the egg yolk and vanilla extract, and beat until just combined.

4. Gradually add the dry ingredients to the wet ingredients and mix on low speed until a dough forms. Do not overmix.

5. Cover the bowl with plastic wrap and chill the dough in the refrigerator for at least 30 minutes.

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

7. Roll chilled dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets.

8. Use your thumb or the back of a small spoon to gently press an indentation into the center of each ball.

9. Bake for 10–12 minutes, or until the edges are set. If needed, re-press the centers immediately after baking.

10. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

11. To make the ganache, heat the heavy cream in a small saucepan until just simmering, then pour over chocolate chips in a bowl.

12. Let sit for 5 minutes, then stir until smooth. Spoon or pipe ganache into the centers of the cooled cookies and let set.