Preheat oven to 180°C. Line a 20 × 20 cm baking pan with parchment paper.
Melt the dark chocolate and butter together until smooth. Let cool slightly.
Beat the eggs and sugar until the mixture becomes lighter in color.
Add the melted chocolate mixture and stir until combined.
Stir in the flour, cocoa powder, salt, and vanilla just until no dry streaks remain.
Pour into the prepared pan and bake for 20–25 minutes. The brownie should stay moist (a toothpick should come out with moist crumbs, not dry).
Cool completely in the pan, then pour the 100 ml coffee evenly over the brownie base.
Whip mascarpone, powdered sugar, and cold heavy cream until smooth and thick. Spread over the cooled coffee-soaked base.
Dust with cocoa powder.
Refrigerate for 2 hours, then slice and serve.