Special equipment: a 9-inch springform pan
For the brownie: Preheat the oven to 350 degrees F. Spray the bottom of a 9-inch springform pan with nonstick cooking spray.
Whisk together the flour, cocoa powder and sugar in a medium bowl. Whisk in the milk, butter, vanilla and egg until smooth. Pour the batter into the prepared pan and bake until a knife inserted in the center comes out clean, about 20 minutes. Cool completely on a wire rack, about 30 minutes.
Unclasp the pan, remove the outer ring and spray it with nonstick cooking spray. Line the ring with a 2 1/2-inch-wide parchment strip, making sure that it sits tightly and smoothly along the sides. Re-clasp the ring to the bottom of the pan with the cake in it.
For the mousse: Roughly chop 4 tablespoons of the butter and put in a medium microwave-safe bowl. Microwave in 30-second intervals, stirring in between each, until melted, about 1 minute. Stir in 1/4 cup of the cocoa powder until smooth; set aside. Chop the remaining 8 tablespoons butter and put in a medium microwave-safe bowl. Microwave in 30-second intervals, stirring in between each, until melted, about 1 minute. Stir in the remaining 1/2 cup cocoa powder until smooth; set aside.
Add 1/4 cup of the heavy cream to a small microwave-safe bowl and stir in the gelatin. Let sit for 5 minutes to bloom. Microwave in 30-second intervals, stirring in between each, until the gelatin is dissolved, about 1 minute. Cool to room temperature, about 5 minutes.
Transfer the gelatin to the bowl of a stand mixer fitted with the whisk attachment and add the sugar, vanilla and remaining 3 3/4 cups heavy cream. Whisk on medium speed until stiff peaks form, 3 to 4 minutes. Divide the whipped cream mixture between 2 large bowls. Fold one bowl of the cocoa powder-butter mixture into each batch of whipped cream mixture and stir until no streaks remain (one bowl will contain dark chocolate mousse and the other will contain light chocolate mousse).
Spread the dark chocolate mousse on top of the brownie in the springform pan. Use an offset spatula to even out and smooth the top. Spread the light chocolate mousse on top of the dark chocolate mousse and use an offset spatula to even out and smooth the top. Refrigerate until the mousse is set, 3 hours and up to overnight.
For the ganache: Bring the heavy cream and butter to a boil in a small saucepan over medium-high heat. Put the semisweet chocolate in a large heatproof bowl and pour the hot cream over top. Let it sit for 5 minutes, then stir until smooth and shiny.
Unclasp the springform pan and remove the parchment paper from the sides of the cake. Use a hot, dry offset spatula to smooth out the sides. Set a rack in a rimmed baking sheet and put the cake on top of the rack. Pour the ganache over the cake and let it drip down over the sides so the entire cake is covered. Swipe once across the top with a clean, large offset spatula to create a smooth top. Refrigerate until set, about 30 minutes.
Hold a stencil or other tool with holes (such as a doily or a cooling rack) over the cake (do not lay it on the ganache; it will stick) and dust with more cocoa powder to create a decorative pattern. Serve immediately.
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