Preheat your oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. Lightly spray the parchment and sides with nonstick spray.

In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until glossy and thick, about 1–2 minutes by hand. Add the eggs and 2 teaspoons vanilla extract, whisking until the mixture looks smooth and slightly lighter in color.

Sift the flour, cocoa powder, 1 teaspoon fine sea salt, and 1 1/2 teaspoons instant espresso powder directly into the bowl. Gently fold with a spatula just until no dry streaks remain. The batter will be thick and shiny; avoid overmixing to keep the brownies fudgy.

Spread the brownie batter into the prepared pan, smoothing the top. Bake for 22–28 minutes, until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.

Set the pan on a cooling rack and let the brownies cool for at least 20–30 minutes. They should be slightly warm but not hot before adding the espresso soak.

In a small bowl or measuring cup, stir together the cooled brewed coffee or espresso, 1–2 tablespoons granulated sugar, and 1–2 tablespoons coffee liqueur (if using) until the sugar dissolves. Taste and adjust sweetness; the coffee should be lightly sweet but still bold.

Using a pastry brush, gently brush the espresso mixture over the surface of the warm brownies. Lightly moisten the top without fully soaking the brownies; you may not need to use all of the mixture. Let the pan sit while you prepare the mascarpone cream.

In a medium bowl, beat the mascarpone cheese and cream cheese together with a hand mixer on medium speed until smooth and lump-free, about 1–2 minutes.

Add the powdered sugar, 1 teaspoon vanilla extract, and 1/4 teaspoon fine sea salt to the mascarpone mixture. Beat again until creamy and well combined.

In a separate cold bowl, whip the heavy whipping cream to medium-stiff peaks. Gently fold the whipped cream into the mascarpone mixture in two additions using a spatula, until smooth, light, and spreadable.

Spoon the mascarpone cream over the cooled, espresso-brushed brownie base. Use an offset spatula to spread it into an even layer all the way to the edges, smoothing the top or leaving gentle swirls.

Cover the pan loosely with plastic wrap and refrigerate for at least 3 hours, or overnight, until the mascarpone layer is firm and the flavors have melded.

Right before serving, dust the top evenly with 1–2 tablespoons unsweetened cocoa powder using a small sieve. Add dark chocolate shavings if desired. Lift the brownies out using the parchment overhang, transfer to a cutting board, and slice into squares with a sharp knife, wiping the blade clean between cuts.