Preheat the oven to 350°F (175°C). Lightly grease an 8-inch square baking pan with cooking spray or softened butter and line with parchment paper, allowing some overhang for easy removal.

In a heatproof bowl set over a saucepan of simmering water (or using the microwave in short bursts), melt the chopped bittersweet chocolate and butter together until smooth and glossy. Stir often, and once melted, remove from heat and allow to cool slightly.

In a large mixing bowl, whisk together the granulated sugar, dark brown sugar, and eggs until the mixture is lightened in color and slightly thickened. Add the espresso powder, vanilla extract, and salt. Whisk until well combined.

Slowly pour in the cooled chocolate mixture, stirring constantly to incorporate. Gently fold in the flour and baking powder until just combined—avoid overmixing to maintain a fudgy texture.

Pour the brownie batter into the prepared pan and smooth the surface with a spatula. Bake for 25–30 minutes or until the center is just set and a toothpick inserted in the middle comes out with a few moist crumbs. Let the brownies cool completely in the pan on a wire rack.

While the brownies are cooling, prepare the tiramisu topping. In a mixing bowl, whisk the mascarpone cheese until smooth. In a separate bowl, beat the cold heavy cream, powdered sugar, vanilla extract, and espresso powder until stiff peaks form.

Gently fold the whipped cream mixture into the mascarpone until light and fluffy. Once the brownies are fully cooled, spread the tiramisu topping evenly across the surface using an offset spatula.

Dust the top generously with unsweetened cocoa powder. Refrigerate for at least 1 hour to allow the topping to set. Slice into squares and serve chilled or at room temperature.