Preheat the oven to 350 degrees. Line a large cookie sheet with parchment paper. Set aside.

In a medium bowl {or the bowl of a stand mixer} beat together butter and sugar until fluffy. Add egg yolk and vanilla extract and mix until fully combined.

Add cocoa powder, flour and salt and mix until the dough comes together.

Using a cookie scoop or spoon, scoop out about 1 ½ tablespoons of dough and roll into balls. Place onto prepared baking pan.

Bake for 14-15 minutes or until the cookies are set. Remove from oven and, using the back of a ½ teaspoon measure, the end of a wooden spoon or your thumb, gently press a well into the center of each cookie. Allow to cool completely.

Make the filling: Place chocolate chips in a small bowl. In another small, microwave-safe bowl, heat the half and half for about 20-30 seconds until just steaming & bubbling around the edges.

Stir in the instant coffee granules and peppermint extract and stir until the granules have dissolved.

Pour the mixture over the chocolate chips and let sit for about a minute, then whisk until smooth and shiny.

Carefully fill the indentations of each cookie with the chocolate. {You'll enough filling for 2 batches of cookies.}

While the filling sets a bit {it will stay soft but shouldn't be runny}, melt about 2 tablespoons of semi-sweet chocolate chips in a small microwave-safe bowl, and whisk until smooth.

Drizzle or pipe over the cookies; allow them to set completely before serving or packaging.