Preheat oven to 350°F (175°C). Line a 9x9-inch pan with parchment with overhang for easy lifting.
Measure and set everything out. Chill the mascarpone and cream; the topping whips best cold.
Melt butter and chopped dark chocolate in a heatproof bowl over a simmering pot or in short microwave bursts. Stir until glossy and smooth.
Whisk in granulated sugar and brown sugar until combined and slightly cooled, 1 minute.
Whisk in eggs one at a time, then vanilla. Beat briskly for 30–45 seconds to create a shiny, thick batter.
Sift in flour, cocoa, salt, and espresso powder. Fold gently with a spatula until no dry streaks remain.
Spread into the lined pan. Tap the pan once to level.
Bake 22–28 minutes until the edges look set and a toothpick comes out with a few moist crumbs. Do not overbake.
Cool the brownie slab in the pan on a rack until just warm.
Stir cooled espresso, liqueur (if using), and sugar until dissolved.
Use a skewer to poke shallow holes across the brownie surface.
Brush or spoon the coffee mixture evenly over the brownies. Let it absorb and cool completely.
In a bowl, whisk mascarpone, powdered sugar, vanilla, cornstarch, and salt just until smooth. Do not overmix.
In a separate chilled bowl, beat heavy cream to medium-stiff peaks.
Fold the whipped cream into the mascarpone mixture in two additions until fluffy and unified.
Spread the mascarpone cream over the cooled, soaked brownie base.
Smooth the top with an offset spatula.
Cover and chill at least 4 hours (overnight tastes best) to set and deepen the flavors.
Right before serving, dust generously with cocoa powder. Add chocolate shavings if you want extra drama.
Lift from the pan using the parchment. Slice with a hot, dry knife for clean edges.
