Cream together the butter, granulated sugar, brown sugar, and salt until lightened. Scrape down he sides and bottom of the bowl to ensure there are no under creamed bits.

Add the egg yolk and vanilla and mix until emulsified and combined. Scrape down the sides and bottom of the bowl.

Add the flour, cocoa powder, and espresso powder. (Sifting the cocoa powder is recommended to prevent lumps.) Mix until combined.

Drop the dough onto a piece of lightly floured parchment paper. Gently form the dough lump into a more tightly formed ball or disc and lightly flour the top. Add another piece of parchment paper to the top and roll the dough to 1/4″. Refrigerate the dough for an hour or two (or up to three days).

Preheat the oven to 375 F (190 C).

Cut shapes from the chilled dough and place on a parchment lined baking sheet a couple inches apart. To really preserve the shapes of the cookies, freeze the sheet for 5 minutes before baking. Bake the cookies until the tops are set and no longer shiny, about 7-9 minutes. *If you want to bake these thin and crunchy, roll to 1/6″ or 1/8″ and bake them for a few minutes longer.

Allow the cookies to cool on the pan for about five minutes before transferring them to a wire rack to cool completely. Decorate with icing or buttercream if desired.