Preheat your oven to 350°F (177°C) and line two baking sheets with parchment paper.

In a large mixing bowl, cream the unsalted butter and granulated sugar until light and fluffy, then add the egg and vanilla extract and mix well.

In a separate bowl, whisk together the flour, cocoa powder, and salt. Gradually add to the butter mixture, stirring until just combined.

Roll the dough into 1-inch balls and place them on the baking sheets. Press a thumbprint in the center of each ball.

Bake for about 12 minutes. Let them cool on the sheets for 5 minutes before transferring to a wire rack.

Combine the chocolate chips and heavy cream in a microwave-safe bowl. Microwave until smooth and glossy.

Fill each cookie indentation with the chocolate mixture and top with holiday sprinkles.