Preheat your oven to 350°F (175°C). In a medium saucepan over low heat, combine the finely chopped dark chocolate and cubed unsalted butter, stirring gently until completely melted and smooth.
In a large mixing bowl, whisk together room temperature eggs with granulated sugar and brown sugar until well combined and slightly frothy. Gradually add in the melted chocolate mixture along with vanilla extract, ensuring everything is blended evenly.
Sift together the all-purpose flour and natural cocoa powder in another bowl to avoid lumps. Slowly fold this dry mixture into the wet ingredients just until combined; be careful not to overmix as this can lead to dense brownies.
Pour the brownie batter into a greased baking pan and spread it evenly with a spatula. Bake in your preheated oven for about 25-30 minutes or until a toothpick inserted into the center comes out mostly clean but still moist.
While the brownies are baking, dissolve espresso powder in hot water along with coffee liqueur if using. Dip each ladyfinger briefly into this mixture before layering them on top of cooled brownies once they have finished baking.
In a large bowl, beat cold mascarpone cheese with whipped cream until smooth and fluffy; gradually add powdered sugar during mixing to ensure sweetness is well incorporated. Spread this luxurious cream over the soaked ladyfingers and dust lightly with cocoa powder before serving chilled or at room temperature.