Preheat oven to 350 degrees and grease a 24-cup mini muffin tin well with baking spray. Set aside.
Beat butter, brown sugar, and granulated sugar together until well combined (using a hand mixer, standing mixer, or by hand).
Add in the egg, mixing until combined.
Add in vanilla extract, baking soda, espresso powder and salt, mixing until combined.
Next, add in graham cracker crumbs, cocoa powder and flour. Mix until well combined.
Using a 1” cookie scoop (about 1 Tbsp) disperse dough evenly between the muffin cups.
Bake for 10 minutes, or until the tops are done and the centers have risen (once you take the cookie cups out of the oven, the centers might sink a bit, which is fine).
Immediately, or within a couple of minutes, press centers down to form a cup shape (I used a tsp to do this). Allow to cool completely before removing from the tin.