Preheat the oven to 350°F and line a baking sheet with parchment paper. Nobody enjoys scraping burnt dough.
Whisk the flour, cocoa, coffee powder, baking soda, and salt in a bowl. Dry ingredients deserve even distribution.
Beat the butter, brown sugar, and granulated sugar until fluffy. If you taste-test it, we won’t judge.
Mix in the egg yolk and vanilla. The dough should be smooth and smell amazing.
Gradually add the dry mixture, stirring until just combined. Overmixing = tough cookies (literally).
Scoop tablespoon-sized dough balls onto the baking sheet. Roll them into smooth spheres like a baking pro.
Press a small indentation in each using your thumb or a spoon. This is where the magic will go later.
Bake for 12 minutes. The edges should be set, but the centers should stay slightly soft.
Cool on the tray for 5 minutes, then transfer to a rack. Time for the filling!