Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, and salt.

In a separate large bowl, cream the butter and granulated sugar until light and fluffy.

Beat in the egg yolk, milk, and vanilla extract until well combined.

Gradually incorporate the dry flour mixture into the wet ingredients until a cohesive dough forms.

Roll the dough into 1-inch diameter balls and place them 2 inches apart on the prepared baking sheet.

Press a thumb or the back of a small measuring spoon into the center of each ball to create an indentation.

Bake for 10 to 12 minutes until the edges are firm and set.

While cookies bake, heat the heavy cream in a small saucepan until simmering, then pour over the chocolate chips in a heat-proof bowl.

Let the cream and chocolate sit for 2 minutes, then stir until a smooth ganache forms.

Remove cookies from the oven and carefully re-press the center indentations if they have puffed up.

Transfer cookies to a wire rack to cool completely before filling each center with the prepared ganache.