Preheat your oven to 350°F (175°C). Grease or line your baking pan with parchment paper.
Melt the butter in a bowl, then mix in the sugar, eggs, vanilla, salt, and cocoa powder. Mix until smooth.
Fold in the flour until just combined. Pour into the pan and smooth the top.
Bake for 20–25 minutes, or until a toothpick inserted comes out with a few moist crumbs.
As the brownies bake, combine hot espresso with sugar and Kahlua. Stir until sugar is dissolved. Set aside to cool slightly.
In a bowl, beat the mascarpone with powdered sugar, vanilla, and egg yolk until smooth and creamy.
After baking, poke holes in the warm brownies using a fork or skewer. Brush the espresso syrup generously over the surface.
Gently spread the mascarpone mixture on top.
Return the pan to the oven for an additional 10–12 minutes until the topping is just set but still slightly jiggly.
Cool the brownies to room temperature, then refrigerate for at least 1–2 hours. Dust with cocoa powder before slicing into squares.
