1. Preheat the oven to 350°F (175°C) and prepare an 8-by-8-inch baking pan by lining it with parchment paper, leaving the edges overhanging for easy brownie removal.
2. Melt unsalted butter in a medium saucepan over medium heat. Remove from heat and mix in cocoa powder until smooth.
3. Stir granulated sugar and brown sugar into the butter-cocoa mixture until fully combined.
4. Add eggs, one at a time, whisking vigorously until smooth and glossy. Stir in vanilla extract.
5. Gradually fold in all-purpose flour and salt using a spatula, being careful not to overmix. If desired, add espresso powder for enhanced chocolate flavor.
6. Pour the brownie batter into the prepared pan, spreading it evenly. Bake for 20-25 minutes or until a toothpick inserted into the center comes out with moist crumbs. Let the brownie layer cool completely.
7. While the brownies cool, begin preparing the mousse layer by softening gelatin in cold water.
8. Heat 1 cup of heavy cream in a saucepan. Add the softened gelatin and stir until completely dissolved. If desired, add espresso powder for deeper flavor.
9. Pour the warm cream mixture over chopped dark chocolate in a separate bowl. Let sit for a minute to melt, then whisk until smooth and glossy.
10. In another bowl, whip the remaining heavy cream until stiff peaks form. Gently fold the whipped cream into the chocolate mixture to create a light and airy mousse.
11. Spread the mousse evenly over the cooled brownie base using a spatula, ensuring an even layer.
12. Refrigerate the brownies for at least 2 hours or overnight to allow the mousse to set completely.