Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the egg and vanilla extract until combined.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Using a spoon or cookie scoop, portion out the dough and roll into 1-inch balls. Place the balls on the prepared baking sheet about 2 inches apart.
Using your thumb, press down in the center of each ball to create an indentation.
Bake the cookies in the preheated oven for 12 minutes, or until the edges are set. The centers will look slightly soft.
While the cookies are cooling, prepare the chocolate ganache. In a small saucepan, heat the heavy cream until it’s just about to simmer. Remove from heat and add the chocolate chips. Let it sit for a minute, then stir until smooth.
Once the cookies are completely cooled, fill each thumbprint with a spoonful of chocolate ganache.
Allow the ganache to set for a few minutes before serving.