Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.

Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.

In a separate bowl, cream the softened butter and brown sugar until light and fluffy — about 2–3 minutes with a hand mixer.

Beat in the egg, vanilla extract, and espresso powder until evenly combined and fragrant.

Gradually fold the dry ingredients into the wet mixture until just combined. Don’t overmix.

Drop rounded spoonfuls (about 1 ½ tablespoons) of dough onto the prepared sheet, spacing them 2 inches apart. Flatten each slightly with your fingers or a spatula.

Bake for 10–12 minutes, until edges are set but centers are still soft. Allow cookies to cool on the sheet for 5 minutes, then transfer to a rack to cool completely.

While cookies cool, make the mascarpone cream: whip mascarpone, powdered sugar, and heavy cream until thick and airy (use chilled cream and bowl). Taste and adjust sweetness.

When cookies are fully cool, spread or pipe a generous dollop of mascarpone on each and finish with a light dusting of cocoa powder. Serve immediately or chill briefly to set the cream.