4 ounces dark or bittersweet chocolate, shaved or very finely chopped
1 cup heavy cream, divided
1 teaspoon instant espresso powder
1 tablespoon granulated sugar
Preheat oven to 350 degrees F. Line an 8-by-8-inch square baking pan with parchment paper.
Melt butter in a microwave safe bowl. Add cocoa powder, sugar, and brown sugar and stir until it forms a thick, grainy paste. Add eggs, vanilla, salt, and espresso powder and whisk until smooth. Fold in flour until just incorporated and no streaks of white remain. Spread into prepared pan and bake for about 25 minutes or until a toothpick inserted near the center comes out with a few moist crumbs attached. Remove from oven and let cool completely.
To prepare mousse topping, in a small dish, sprinkle gelatin over 1 tablespoon of cold water and let sit for 5 minutes to soften. Place shaved chocolate in a heat-proof bowl.
In a small saucepan, warm 1/3 cup of the heavy cream until steaming. Whisk in softened gelatin and espresso powder until dissolved. Pour warm cream over shaved chocolate. Let sit for 20 to 30 seconds, then gently whisk until smooth.
Place remaining 2/3 cup heavy cream and granulated sugar in a large bowl or the bowl of a stand mixer fitted with the whisk attachment. Whip on medium-high speed until it forms soft peaks.
With a large rubber spatula, fold in half of whipped cream to chocolate mixture to lighten. Add remaining cream and gently fold, scraping the sides and bottom of the bowl, until mousse is uniform in color.
Spoon mousse on top of cooled brownies, and smooth into an even layer with an offset spatula. Do this fairly quickly, as the gelatin will begin to set up fairly quickly and the mousse will be much harder to spread smoothly.
Lightly cover with plastic wrap and refrigerate until set, at least 2 hours or overnight (if possible).
To cut, lift out entire block using the parchment paper as handles. Cut into a 4x4 grid of squares (or smaller if you prefer) using a sharp knife, wiping off knife between cuts. If desired, drizzle with melted chocolate for garnish before or after cutting into squares.