For the chocolate mousse: In a cold metal or glass bowl, whip the cream to soft peaks. Set aside.
Fill a saucepan about a third of the way with water and bring to a simmer. Put the chocolate and butter in a medium heatproof bowl and place over the simmering water, making sure that the bottom does not touch the water. Heat, stirring, until melted and smooth. Set the mixture aside. Keep the pot of water simmering but not boiling.
Put the whole egg, yolks and salt in a heatproof bowl. Beat with an electric mixer until light, about 1 minute. Slowly add the granulated sugar to the egg mixture while continuing to beat. Set the bowl over the simmering water and increase the heat to bring the water to a rapid simmer. Whisk the egg mixture constantly over the heat until pale yellow, very fluffy and hot to the touch, 3 to 5 minutes. Remove from the heat and whip until cool and thick and the mixture folds over on itself (holds a ribbon), about 3 minutes.
Whisk about a quarter of the egg foam into the chocolate to lighten. With the whisk, briskly fold in the remaining foam (the mixture may get very thick). Whisk in the whipped cream, taking care to keep the mixture light. Hold in the refrigerator.
For the frosting: Combine the confectioners' sugar, butter and salt in the bowl of a stand mixer fitted with a paddle attachment (or in a large bowl if using a handheld electric mixer). Mix on low speed until mostly incorporated. Add the vanilla, increase the speed to medium high and mix until smooth. Adjust the consistency with the milk as desired. Mix in enough of the red food coloring to make the frosting cotton-candy pink. Transfer to a piping bag fitted with a medium plain tip.
For the rice cereal treat shell: Add the marshmallows and butter to a large pot over medium-low heat and cook, stirring, until melted. Add the rice cereal and mix to combine.
While the cereal mixture is warm, spray the inside of a medium bowl and your hands with nonstick spray. Line the bowl with a 2-inch-thick layer of the cereal mixture. Spray the outside of a smaller bowl with nonstick spray and place it in the center of the cereal shell, pressing down to compress and even out the cereal layer. Once the cereal has set, spoon the chocolate mousse into the center of the shell. Flatten the remaining cereal mixture and lay it over the mousse, pressing the sides to seal. Refrigerate until chilled, about 15 minutes.
Invert onto a platter or baking sheet and form gently to look like a brain. Pipe the frosting on top of the cereal shell in a brain-like pattern. Make a 1-inch line down the center of the brain with a butter knife or offset spatula and top the frosting line with red sprinkles.
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