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1
Use a serrated knife to cut the domed top off of both cakes, then to cut each into two even layers. You'll have four layers total.
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2
Cut a piece of wax or parchment paper to the size of a baking sheet. Place a 9-inch cake ring (or the outside of a 9-inch springform pan) on top of the paper, and set aside.
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3
Make the mousse: Place the water in a small bowl, and sprinkle the gelatin over the surface. Let bloom for 5 minutes.
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4
In a medium, heat-safe bowl, whisk the egg yolks and half the sugar (1/2 cup) to combine. Place over a pot of simmering water and whisk until the mixture reaches 140u00b0 F on a thermometer. Return the bowl to the stand mixer (or use an electric hand mixer) and whip on medium speed until you reach full volume/ribbon stage (the mixture will be pale yellow and fall back into the bowl in ribbons, which disappear slowly disappear). Set aside the egg yolk mixture and clean the bowl of the stand mixer.
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5
In a separate bowl, whip the cream to medium peaks and set aside. Ready the melted, cooled chocolate by transferring it to a large bowl. Melt the gelatin; I usually just pop it in the microwave for 10 to 15 seconds. Stir the gelatin into the chocolate.
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6
In the bowl of the stand mixer, whisk together the egg whites and remaining 1/2 cup sugar. Place the bowl over a medium pot of barely simmering water and continue to whisk by hand until the mixture reaches 140u00b0 F on a thermometer. At this point, return the bowl to the stand mixer and whip on medium speed until you reach medium peaks.
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7
Gently fold the whipped egg yolks into the chocolate. Fold until fully combined, but try to work quickly to avoid deflating the foam. Next, gently fold in the whipped cream.
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8
Finally, fold in the egg whites, working quickly to avoid deflating the foam, but making sure it's fully combined.
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9
Ladle an even layer of mousse into the center of the ring on the prepared pan-about 2 inches of mousse, but no need to be exact. This will eventually be the top of the cake!
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10
Press a cake layer gently into the mousse-don't press it down so hard that it hits the pan, just nestle it into the mousse a little. Ladle another layer of mousse on top (you can use the number of ladlefuls to help approximate even layers).
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11
Place another layer of cake on top and keep building, layering mousse and cake. When you press the final layer of cake in, you should be close to the top of the ring. This will be the base of the cake!
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12
When you press the last layer down, the mousse will flood up the edges. Ladle a little more mousse on top of the cake, and use an offset spatula to smooth it, creating a flat surface from the edge of the pan all the way across.
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13
Place the cake (inside the ring on the pan) into the freezer (you an also put it in the refrigerator, but freezing at this point makes unmolding easier). Chill until totally set.
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14
While the cake chills, make the glaze. Place the cool water in a small bowl, and sprinkle the gelatin over it. Bloom for 5 minutes.
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15
In a small bowl, whisk the sugar and cocoa to combine. Transfer the mixture to a small pot with the cream. Bring to a simmer over medium heat and simmer, stirring occasionally, until the mixture thickens, 12 to 15 minutes. Stir in the melted gelatin.
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16
Transfer the glaze to a heat-safe vessel with a spout (like a liquid measuring cup) and cool until no longer warm to the touch, but still fluid (about 85u00b0 F-ish).
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17
When the cake is chilled, remove it from the freezer. Use a kitchen torch to warm the outside of the ring, then lift it off the cake. Invert the cake onto a cooling rack to glaze it. You can use the paper to help lift the cake up and turn it over; if the cake is properly chilled, this should be easy! Once it's inverted, peel the paper away.
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18
Ladle the cooled glaze over the cake, letting the excess drain off the rack. Before the glaze sets (which could happen pretty quickly because the cake is cold!), decorate with some sprinkles at the top and base.
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19
Lift the cake off the cooling rack and transfer to a platter or cake stand. Let it come to room temperature before you serve, or store in the refrigerator until ready to serve.
