Line a square baking pan with parchment and preheat the oven. Whisk the flour and salt together in a small bowl. Melt the butter and chocolate chips, then whisk in the cocoa and espresso powders. Beat the sugars, eggs, yolk, and vanilla until the mixture is pale and light. Incorporate the melted chocolate mixture followed by the dry ingredients, then bake until a toothpick comes out with moist crumbs.

Cool the brownies completely in the pan, using the freezer to speed up the process if needed. Dissolve sugar into the hot coffee to create the soak. Heat the egg yolks and sugar over a water bath until the sugar dissolves, then let cool. Beat the mascarpone into the yolks before adding cold heavy cream and vanilla, whisking until stiff peaks form.

Brush the brownie base with coffee and spread a thin layer of cream. Quickly dip ladyfingers into the coffee soak and layer them across the brownie. Spread the remaining mascarpone cream on top, dust with cocoa powder, and chill for at least two hours before slicing with a clean knife.