Preheat the oven to 175°C / 350°F and line a 6-inch square baking pan with parchment paper.

Whisk the caster sugar, brown sugar, and egg until the mixture becomes light and pale.

Melt the dark chocolate and butter together until smooth, using short microwave bursts and stirring between each burst.

Pour the melted chocolate mixture into the egg mixture and whisk until fully combined.

Fold the all-purpose flour and 8 g cocoa powder into the batter until no dry streaks remain.

Spread the brownie batter evenly into the lined baking pan.

Bake the brownie for about 18 to 20 minutes, until the edges are set and the center remains slightly fudgy.

Remove the brownie from the oven and let it cool completely in the pan.

Whisk the mascarpone cheese and icing sugar together until smooth and lump-free.

Stir the vanilla extract and whipped cream into the mascarpone mixture until the filling is smooth and airy.

Briefly dip all 8 ladyfinger biscuits in the brewed coffee, ensuring they are soaked but not falling apart.

Arrange the coffee-soaked ladyfinger biscuits in an even layer over the cooled brownie.

Spread the mascarpone filling evenly over the ladyfinger layer.

Dust the surface generously with the cocoa powder for dusting.

Refrigerate the assembled brownie tiramisu for 3 to 4 hours, until the mascarpone layer is firm and set.

Slice the chilled dessert into 4 portions and serve cold.