1. Preheat oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper.
2. Whisk flour and salt; set aside. Melt butter with chocolate over low heat. Whisk in cocoa and espresso powders.
3. In a separate bowl, whisk sugars, eggs, yolk, and vanilla until fluffy.
4. Fold melted chocolate into egg mixture, then gently fold in flour mixture.
5. Pour batter into prepared pan, bake 18-25 minutes until moist crumbs cling to a toothpick.
6. Cool brownies, then make espresso soak by dissolving sugar in hot espresso, adding liqueur.
7. For mascarpone filling, heat egg yolks and sugar over simmering water, whisking until thickened at 145°F. Cool.
8. Beat yolk mixture with mascarpone. Whip cream with vanilla to stiff peaks, fold into mascarpone.
9. Brush brownies with espresso soak, spread a layer of mascarpone cream.
10. Dip ladyfingers briefly in espresso, layer over mascarpone, spread remaining cream.
11. Dust with cocoa powder, refrigerate 1-2 hours before slicing.