Chocolate Espresso Sugar Cookies

Adjust an oven rack to the middle of the oven. Preheat the oven to 350F [180C]. Line three sheet pans with parchment paper.

In a small bowl, combine the flour, cocoa powder, baking soda, and cream of tartar.

In the bowl of a stand mixer fitted with a paddle, beat the butter on medium speed until creamy, about 1 minute. Add the granulated sugar, espresso grounds, and salt, and beat again until light and fluffy, 2 to 3 minutes. Add the egg, yolk, vanilla, and Kahlua (if using), and beat on low speed until combined. Add the flour mixture and beat again on low speed until combined. Add the chopped chocolate and mix on low until just combined.

Place some sanding sugar in a medium bowl.

Scoop the dough into 1 ½ oz [45 g] portions, about 2 tablespoons each, and roll into a ball. Roll each ball in the sugar. Place 8 cookies on each sheet pan. Sprinkle with flaky salt if desired.

Bake one pan at a time, rotating halfway through baking. Bake until the sides are set and the top beings to crackle, 12 to 14 minutes. Transfer the sheet pan to a wire rack and let the cookies cool for 5 to 10 minutes on the pan, then remove the cookies and let them cool completely on the wire rack. Store cookies in an airtight container at room temperature for up to 3 days.