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

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt.

In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy. Mix in the vanilla extract and egg yolk.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Form the dough into 1-inch balls and place them on the prepared baking sheet, spaced 2 inches apart.

Press down in the center of each cookie to create an indentation.

Bake for 10-12 minutes, until the edges are set but the centers remain soft.

While warm, press chocolate chips into the center of each thumbprint and allow to cool completely on a wire rack.