Cream the butter and sugars until light and fluffy for about 3-4 minutes.

Combine the egg, vanilla extract, sourdough discard, and espresso powder into the butter-sugar mixture, mixing until smooth.

Whisk together the flour, cocoa powder, baking soda, and salt in a separate bowl.

Gradually add the dry mixture to the wet mixture, folding gently until no flour is visible.

Chill the dough in the refrigerator for at least 30 minutes.

Scoop the dough onto a baking tray lined with parchment paper, leaving space between each cookie.

Bake in a preheated oven at 350°F (175°C) for 10-12 minutes.

Let the cookies cool on the tray for 5 minutes before transferring to a wire rack.