Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the long sides to form a sling. Lightly grease the parchment and exposed sides of the pan.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until glossy and smooth, about 1 minute. Add the eggs one at a time, whisking well after each addition, then whisk in the vanilla extract until the mixture is thick and shiny.

In a separate bowl, whisk together the cocoa powder, flour, salt, and instant espresso powder until well combined. Gently fold the dry mixture into the wet ingredients with a spatula just until combined. Fold in the chocolate chips, if using. The batter will be thick.

Spread the brownie batter evenly into the prepared pan and smooth the top. Bake for 22–28 minutes, or until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter. Place the pan on a cooling rack and let the brownies cool completely to room temperature in the pan.

While the brownies cool, stir together the hot brewed espresso or strong coffee, coffee liqueur (if using), and sugar in a small bowl until the sugar dissolves. Let the mixture cool to room temperature, then taste and adjust sugar as needed for a bold but pleasant coffee flavor.

Once the brownie layer is fully cool, use a fork or skewer to gently poke small holes all over the surface, without piercing all the way through. Using a pastry brush or spoon, evenly drizzle and brush the cooled espresso mixture over the brownies so it soaks in lightly but does not saturate them.

In a medium bowl, beat the mascarpone cheese and cream cheese together with a hand mixer on medium speed until smooth and creamy, 1–2 minutes. Do not overbeat.

Add the powdered sugar and vanilla extract to the mascarpone mixture and beat again until fully combined and silky.

In a separate chilled bowl, whip the cold heavy cream to soft–medium peaks. Gently fold the whipped cream into the mascarpone mixture with a spatula until no streaks remain and the cream is smooth, fluffy, and spreadable.

Spread the mascarpone cream evenly over the cooled, espresso-brushed brownie layer. Use an offset spatula to smooth the top. Cover the pan lightly with plastic wrap and refrigerate for at least 3–4 hours, or overnight, until the mascarpone layer is set and chilled.

Just before serving, sift unsweetened cocoa powder generously over the top. Add dark chocolate curls or finely grated chocolate if desired. Use the parchment sling to lift the brownies from the pan onto a cutting board, then cut into 16 larger squares or up to 24 smaller bars, wiping the knife clean between cuts for sharp layers.