Easiest Chocolate Mousse Cake

Directions

1. Make the brownie layer: Heat the oven to 325°F. Grease the bottom and sides of a 9-inch springform pan (at least 3 inches in height) with nonstick spray or softened butter. Line the bottom with a round of parchment paper and the sides with a long strip.
2. In a small bowl, whisk the cocoa, flour, and salt. Whisk in the nuts.
3. In a large bowl, vigorously whisk the brown sugar, melted butter, and vanilla. Whisk in the eggs. Fold in the dry ingredients with a flexible spatula, just until the last streak of flour disappears.
4. Transfer the batter to the prepared pan and bake for about 35 minutes, rotating the pan at the halfway point. It’s ready when a cake tester comes out with a few moist, almost wet, crumbs that can be rolled into a ball between your fingers.
5. Turn off the oven and let the brownie cool on a rack on the counter (or in the refrigerator, if you’re feeling impatient) until it’s room temperature. (At this point, you can also wrap the brownie and wait for up to 24 hours to proceed to the next step.)
6. When the brownie is room temperature, make the mousse: Whisk the hot tap water and cocoa in a small bowl. In a large heatproof bowl set over a pot of simmering water, melt the chocolate (or use a microwave if you prefer).
7. In the bowl of a stand mixer fitted with the whisk attachment, beat the cream, salt, and vanilla on medium-high speed until the cream thickens but does not hold peaks. When you lift the whisk, the cream should form ribbons as it falls and hits the surface.
8. Stir the cocoa mixture into the chocolate with a flexible spatula. Gently fold in the cream until just a few streaks of cream remain (no need to clean this bowl—we’re using it again soon). Gently fold in the raspberries. Scrape the mousse into the pan and evenly spread it over the cooled brownie. Transfer to the refrigerator.
9. Make the whipped cream: In the stand mixer bowl fitted with the whisk attachment, whisk the cream, vanilla, and sugar on medium-high speed until stiff peaks form. Spread the cream over the mousse layer and refrigerate for at least 2 hours and up to 24. If refrigerating overnight, very lightly cover the cake in plastic wrap.
10. When ready to serve, remove the sides of the pan and peel away the parchment. Slide a long, thin knife between the bottom of the pan and the parchment covering the brownie bottom and release the cake, placing the cake on a serving plate, and carefully peeling away the paper as you do so.
11. For the cleanest slices, dip a knife in warm water before cutting. Serve with more raspberries and cocoa if you’d like. The cake will keep lightly wrapped in the refrigerator for up to 3 days.