In a small bowl, add the cocoa powder to the flour by whisking it together; set aside. Using a paddle attachment on your mixer on low speed, beat the butter and confectioner's sugar until smooth, about 1-2 minutes, stopping to scrape the sides of the bowl as needed. Add in the espresso, vanilla, and salt and beat until combined.

Add the flour/cocoa powder mixture and mix on low speed until combined (about 3 minutes), making sure not to overmix the dough.

Dump the dough onto a piece of plastic wrap and flatten into a disk. Chill for 30-60 minutes (or until the dough is firm enough to work with).

On a lightly floured surface, roll the dough to about 1/4 inch thick. Make sure you're rolling it out evenly so the cookies will bake evenly.

Using a heart or other shape cookie cutter, cut out shapes as close to one another as possible. Use the scraps to roll out and cut more cookie shapes. Chill dough a little bit more if it becomes sticky. Arrange the cookies on two parchment-lined baking sheets and refrigerate until chilled, at least 20 minutes.

Preheat oven to 350 F.

Bake the cookies until golden on the bottom and edges and pale to golden on top, 13-18 minutes.

Cool and spread on some melted chocolate or Nutella. Add Maldon salt or sprinkles.