Melt the butter in a small bowl and whisk in the espresso powder. Chill for 10 minutes.

In a separate bowl, whisk flour, cornstarch, baking powder, baking soda, and salt until combined.

In a large bowl, whisk cooled espresso butter with sugar. Add egg and vanilla and mix until smooth.

Stir dry ingredients into wet ingredients in batches until just combined.

Preheat oven to 375°F and line a baking sheet with parchment. Let dough rest for 30 minutes.

Scoop dough using a 2-tablespoon scoop and space cookies 3 inches apart.

Bake for 10–12 minutes until edges are set and centers are slightly puffed.

Cool cookies completely before frosting.

Whisk mascarpone on low until smooth. Add espresso powder, vanilla, and powdered sugar.

Slowly add heavy cream, then whip on medium until light and fluffy.

Frost cooled cookies and dust with cocoa powder before serving.