Preheat & Prep: Preheat the oven to 350°F and line a baking sheet with parchment paper.
Make the Ganache: In a microwave-safe bowl, heat the heavy cream for 1 minute. Add in the chocolate chips and stir until smooth and fully combined. Set aside to cool. You can also prepare this in a double boiler for a silkier texture.
Prepare the Cookie Dough: Using a stand mixer with a paddle attachment (or a hand mixer), cream the softened butter, brown sugar, and granulated sugar until light and fluffy.
Add in the egg yolk and vanilla extract, mixing until well incorporated.
Slowly mix in the flour until the dough starts coming together. If using, fold in the nonpareils sprinkles for an extra festive look.
Form the Cookies: Scoop about 1 ½ tablespoons of dough, roll it into a ball, and place it on the prepared baking sheet.
Use the back of a tablespoon to make an indent in the center of each cookie. If the dough feels sticky, lightly flour your hands.
Chill: Place the formed cookies in the freezer for 5 minutes to help maintain their shape while baking.
Bake: If using chocolate ganache, bake the cookies without the filling for 12 minutes. The centers may puff up slightly—press them down again gently after baking.
If using jam, fill the indentations before baking and then bake for 12 minutes.
Cool & Fill: Let the cookies cool completely before filling with chocolate ganache. Once filled, allow the ganache to set for a few hours before stacking or gifting.
Enjoy!