Preheat the oven to 350°F (175°C).
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the egg and vanilla extract, mixing until well combined.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Chill the dough for about 30 minutes in the refrigerator.
Scoop tablespoon-sized portions of dough and roll them into balls.
Place the dough balls on a baking sheet lined with parchment paper, leaving space between each.
Use your thumb or the back of a spoon to create an indentation in the center of each dough ball.
Fill each indentation with a small amount of chocolate chips or jam.
Bake in the preheated oven for 10-12 minutes, or until the edges are set.
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack.
Enjoy your delicious Chocolate Thumbprint Cookies!