Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a bowl, whisk together flour, cocoa powder, baking soda, and salt. In another bowl, beat softened butter and sugar until light and fluffy.
Mix in the egg, vanilla extract, and espresso powder into the butter mixture until well combined. Gradually stir in dry ingredients until just combined.
Drop tablespoon-sized portions onto the prepared baking sheets, spacing them two inches apart. Bake for 10–12 minutes until edges are set but centers remain soft.
Allow cookies to cool on the baking sheets for five minutes before transferring to wire racks to cool completely.
For the filling, whip together mascarpone cheese and additional espresso powder with powdered sugar until smooth. Spread between two cooled cookies.
