Begin by creaming the softened unsalted butter and granulated sugar together in a large mixing bowl using a hand mixer until light and fluffy, about 3-5 minutes.

Beat in a large egg, vanilla extract, and coffee extract if desired, mixing until well combined and creamy.

In a separate bowl, sift together plain flour, cocoa powder, and salt. Gradually add this dry mixture to the butter mixture, folding gently until just combined.

Scoop out tablespoon-sized portions of the cookie dough and roll into balls, placing them on a baking sheet lined with parchment paper.

Use your thumb or the back of a spoon to create an indent in the center of each ball.

Refrigerate the baking sheet for 30-40 minutes.

Preheat your oven to 180°C (165°C fan) and bake the cookies for about 10 minutes.

Prepare the ganache by heating double cream until just boiling, then pouring it over the chopped chocolate and stirring until smooth.

Fill each indent in the cooled cookies with the ganache and sprinkle with festive sprinkles.