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1
Make the ganache: Place the chocolate in a large, heat-safe bowl. In a medium pot, bring the cream to a simmer over medium heat.
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2
Pour the hot cream over the chocolate and let sit, undisturbed, for 15 to 20 seconds. Using a silicone spatula, begin stirring in tight circles in the center of the bowl, eventually working in larger circles outward until the ganache is smooth and glossy. Set aside to cool to a spreadable consistency (20 to 30 minutes)-you can use the fridge or freezer to speed up the process, just keep an eye on it; you don't want it to be liquid anymore or look oozy, but you also don't want it to be hard or difficult to work with. Ideally, it should be spreadable, like thick frosting!
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3
The marshmallow needs to be used right away once it is made, so before you begin to mix it, get everything ready to assemble the cake. Working one cake layer at a time, scoop 1 cup of ganache into the center of the cake, and spread into an even layer-the ganache will fill the slightly sunken cavity of the cake, but continue to spread it nearly to the edges. Repeat on all cake layers, and set aside. Transfer the remaining ganache to a disposable pastry bag and cut a 1/4-inch opening from the tip. Set aside.
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4
Grease a silicone spatula with nonstick spray. Grease the inside of a disposable pastry bag with nonstick spray. You can fit it with a large round tip, or just cut a 1/2-inch opening from the end. I find it can be handy to have a pair of kitchen scissors handy-spray them with nonstick spray too!
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5
Pour 1/3 cup cool water into a medium, shallow dish and sprinkle the gelatin even over the surface. Let bloom for 5 minutes.
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6
Pour the remaining 1/3 cup water and the sugar into a medium pot. Heat the pot over medium-high heat. Stir the mixture until it comes to a boil, then stop stirring completely: Agitating a boiling sugar syrup can encourage crystals to form, which can lead to disaster. Once you stop stirring, brush any visible sugar crystals away from the sides of the pot with a damp pastry brush.
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7
Once the mixture begins to boil, fasten a candy thermometer to the side of the pot. Continue to boil until the temperature reads 245u00b0 F.
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8
Pour the syrup into the bowl of an electric mixer fitted with the whisk attachment. Add the vanilla and let the mixture cool to 220u00b0 F.
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9
While the syrup cools, melt the bloomed gelatin, either in the microwave (15 to 20 seconds) or over a double boiler, until it is fluid.
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10
When the sugar syrup has cooled, add it to the bowl of your mixer and gradually work the mixer up to medium speed. Once the mixer is running, add the melted gelatin (and any extracts, if using). Whip the sugar on medium speed until the bowl feels almost entirely cool to the touch and the sugar is opaque white and very fluffy, 4 to 5 minutes. It should hold stiff peaks. Try not to stop and restart the mixer multiple times, just keep whipping steadily for 4 to 5 minutes.
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11
While the marshmallow whips, place one layer of the cake on a cake turntable (or straight onto a cake stand). Have the other two layers ready to go.
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12
When the marshmallow is fully aerated (it should hold peaks), use the greased spatula to transfer it to the greased pastry bag. Starting in the center of the first cake layer, pipe a spiral of marshmallow, working to the outside edge. stop about 1/4 inch from the edge. If desired, toast the marshmallow layer with a kitchen torch, then top with the next cake layer. Repeat this process with another cake layer, then place the final cake layer on top.
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13
For the top of the cake, pipe little rounds/drops of marshmallow all over the top of the cake. If you want, use the scissors to help snip the end of each piped piece to help release it from the bag. Once the surface is totally covered, toast it all over with a torch.
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14
Drizzle the remaining ganache over the surface of the cake. Let the cake stand 15 minutes to 20 minutes before slicing and serving.
