Preheat the oven to 350°F. Line an 8x8-inch square baking pan with parchment paper, leaving a few inches hanging over the edges. Lightly grease the pan and parchment with cooking spray or butter.

In a medium saucepan, combine the chopped bittersweet chocolate and unsalted butter. Heat on low, stirring often, until melted and smooth, which should take about 2 to 3 minutes. Remove from heat.

Stir in the granulated sugar and dark brown sugar until blended; the mixture will be textured. Incorporate the eggs one at a time, whisking after each addition. Add the instant espresso powder, vanilla extract, and kosher salt, mixing until smooth.

Mix in the all-purpose flour and baking powder until there are no visible flecks of flour. Pour the batter into the prepared baking pan and spread it evenly.

Bake for 25 to 30 minutes, until a toothpick inserted in the center comes out with a few crumbs. The edges should be firm while the middle remains moist but not overly wet. Allow the brownies to cool completely in the pan for about 3 hours.

In a medium bowl, use a hand mixer on medium speed to beat the room temperature mascarpone cheese and cold heavy cream until soft peaks form. Gradually add the powdered sugar, vanilla extract, and instant espresso powder, continuing to beat until the mixture thickens to stiffer peaks.

Spread the mascarpone mixture evenly over the cooled brownies. Using the overhanging parchment, lift the brownies from the pan and transfer them to a cutting board. Dust the top with unsweetened cocoa powder through a fine mesh strainer. Cut the brownies into 12 to 16 squares, cleaning the knife between cuts for tidy slices.