Preheat your oven to 350°F (175°C).
In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and baking powder.
Cream the softened butter and granulated sugar together until light and fluffy.
Add the vanilla extract to the butter mixture and mix until well combined.
Gradually add the dry mixture to the wet ingredients and stir until just combined.
Scoop out tablespoon-sized portions of dough and roll them into balls. Place them on a baking sheet lined with parchment paper.
Press down in the center of each dough ball to create a thumbprint.
Spoon a small amount of jam or fruit preserves into each thumbprint.
Bake for 12-15 minutes until set but still soft in the center.
Let the cookies cool on the baking sheet for a few minutes, then transfer to a wire rack to cool completely.