Preheat the oven to 350°F (175°C) and prepare a baking sheet.

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

In a separate bowl, cream the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add in the egg yolk and vanilla extract to the butter mixture, mixing well.

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

Scoop tablespoon-sized balls of dough and roll them in your hands to form smooth balls.

Place the balls on the prepared baking sheet, spacing them about 2 inches apart.

Use your thumb or the back of a spoon to create an indentation in the center of each cookie ball.

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

Remove from the oven and let them cool on the baking sheet for a few minutes before transferring.

Once cooled, fill each indentation with your desired filling and let set.