For the cake: Preheat the oven to 350 degrees F. Spray an 8-inch square glass baking dish with nonstick cooking spray.
Whisk together the flour, sugar, cocoa powder, baking soda, baking powder and salt in a large bowl. Whisk in the milk, vegetable oil, vanilla, egg and 1 teaspoon hot water until smooth and combined.
Pour into the prepared baking dish and bake until a knife inserted in the center comes out clean, about 20 minutes. Cool for 10 minutes on a wire rack, then flip out onto a cutting board to cool completely, about 20 minutes.
Use a 2 1/2-inch round biscuit cutter to cut out 6 circles from the cake (save the scraps for another use); set aside.
For the chocolate domes and mousse: Meanwhile, add the coconut oil and 3 ounces of the chocolate to a medium microwave-safe bowl. Microwave in 1-minute intervals, stirring in between each, until the chocolate and coconut oil are melted and the mixture is smooth, about 2 minutes. Pour the chocolate mixture into the cavities of a silicone half-sphere mold with six 3-ounce-capacity cavities and tilt the mold to swirl the chocolate around until each half-sphere is coated entirely (you can also use your finger to swirl the chocolate around). Some chocolate may spill over on top of the mold, which is fine; it will break off when you unmold the domes. Tap out any excess chocolate and freeze until the chocolate is hardened, about 5 minutes.
Put 1/4 cup of the cream in a small bowl and stir in the gelatin. Put the butter and remaining 6 ounces chocolate in a medium microwave-safe bowl and microwave in 1-minute intervals, stirring in between each, until the chocolate and butter are melted and the mixture is smooth, about 2 minutes. Stir the gelatin mixture into the chocolate mixture until the gelatin is dissolved. Cool to room temperature, about 10 minutes.
Whip the remaining 3/4 cup cream in a large bowl with an electric mixer on medium speed until stiff peaks form, 2 to 3 minutes. Add half of the chocolate-gelatin mixture and continue beating until well combined. Add the remaining chocolate-gelatin mixture and continue beating until no streaks remain.
For the filling and topping: Scoop 1/3 cup of the mousse into each chocolate cavity in the half-sphere mold. Using the back of a teaspoon, make a divot in the middle of each portion of chocolate mousse about halfway down. Pour a heaping teaspoon of caramel sauce into each divot. Top each half-sphere with a cake round, pressing down gently into the mousse (use a paper towel to wipe away any mousse that may seep out of the mold). Freeze until the mousse is set, about 1 hour.
Gently peel the mold away from each of the half-spheres and place them cake-side down on a serving platter.
Use a vegetable peeler to shave 1 tablespoon of chocolate curls from the chocolate. Put the remaining chocolate in a small microwave-safe bowl and microwave in 30-second intervals, stirring in between each, until melted, about 1 minute. Drizzle the melted chocolate over each dome and sprinkle with the chocolate shavings. Serve immediately.
a 2 1/2-inch round biscuit cutter; a 6-cavity silicone half-sphere mold (with 3-ounce cavities)
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