Heat oven to 350 degrees F and line a baking sheet with parchment paper.
In a large mixing bowl, add melted butter, granulated sugar, and espresso powder. Whisk together until smooth.
1/2 cup + 2 tbsp butter, 3/4 cup granulated sugar, 2 tbsp instant espresso powder
Add egg and vanilla extract, and whisk until creamy.
1 large egg, 1 tsp vanilla extract
Add flour, cornstarch, baking soda, and sea salt, and stir until just combined and a soft dough is formed.
1¾ cup gluten free all-purpose flour, 1 tsp cornstarch, 1 tsp baking soda, 1/4 tsp sea salt
Scoop 2 tablespoons of dough, and drop on the baking sheet 2-3 inches apart. Bake for 9-12 minutes until the edges begin to set and the centers look puffy and underbaked.
Remove from the oven and cool for 10 minutes, then transfer to cooling rack to cool completely.
In a large bowl, add all ingredients for the frosting. Use an stand mixer or a hand mixer to whip on medium until thick, light, and fluffy.
4 oz mascarpone, 1/2 cup heavy cream, 3/4 cup powdered sugar, 1/2 tsp vanilla extract, 1/2 tsp instant espresso powder
If you want, transfer the frosting to a piping bag with a round tip. Pipe the tops of the cooled cookies in a swirl, starting at the center and moving outward. You can also just frost the cookies with an offset icing spatula if you don't want to use a piping bag. Dust the cookies with cocoa powder through a fine mesh sieve. Enjoy!
Cocoa powder