Preheat your oven to 350°F (175°C).
In a bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a separate mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the egg, vanilla extract, and red food coloring to the creamed mixture, mixing until well combined.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Scoop tablespoon-sized amounts of dough and roll into balls.
Place the dough balls on a baking sheet lined with parchment paper, spacing them about 2 inches apart.
Use your thumb or a small spoon to make an indentation in the center of each cookie ball.
Fill each thumbprint with your chosen filling, being careful not to overfill.
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 a few minutes before transferring them to a wire rack.
