Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a large bowl, cream the softened butter, granulated sugar, and powdered sugar until light and fluffy.
Add the egg yolk and vanilla extract. Mix until well combined.
Add the flour, cocoa powder, and salt. Mix until a soft chocolate dough forms.
Roll the dough into 1-inch balls and arrange them on the prepared baking sheet.
Press your thumb into the center of each dough ball to create a deep well.
Bake for 10–12 minutes, until the cookies are set but still soft.
Gently reshape the centers while the cookies are warm if needed. Allow them to cool completely.
Heat the heavy cream until steaming. Pour it over the chocolate chips and let sit for 2 minutes.
Stir until the chocolate ganache is smooth, silky, and glossy.
Spoon the ganache into the center of each cooled cookie.
Decorate with red, green, and white Christmas sprinkles. Allow the ganache to set before serving.
