Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a mixing bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
In a separate bowl, cream the softened butter with granulated sugar and brown sugar until light and fluffy using an electric mixer or whisk.
Add the egg yolk and vanilla extract to the creamed mixture and mix until well combined.
Gradually add the dry ingredient mixture to the wet ingredients, mixing until the dough forms.
Using a spoon or cookie scoop, form small balls (about 1 inch in diameter) from the dough and place them on the prepared baking sheet, spacing them about 2 inches apart.
Using your thumb or the back of a spoon, create a small indentation in the center of each dough ball.
Bake the cookies in the preheated oven for 10-12 minutes. They should be set but still soft. Remove from the oven and allow to cool for a few minutes on the baking sheet.
While the cookies cool, melt the chocolate chips in a small saucepan over low heat, stirring until smooth.
Once the cookies are slightly cooled, fill each indentation with a small amount of melted chocolate using a spoon.
Transfer the cookies to a wire rack to cool completely before serving.