Special equipment: an 8 1/2-inch metal bowl
Line an 8 1/2-inch metal bowl with plastic wrap, leaving a 4-inch overhang on all sides. Line 2 baking sheets with parchment. Put the bowl and baking sheets in the freezer until chilled.
Have a bowl of warm water on the counter to warm the ice cream scoop for smoother scooping. Working quickly with a 1/4-cup scoop, make smooth scoops of the vanilla and strawberry ice cream and raspberry and mango sorbet, rinsing the scoop well between each pint to keep the colors distinct. Arrange the scoops on the chilled baking sheets flat-side down and freeze until hard, about 30 minutes.
Make the ice cream bomb: Line the chilled bowl with the ice cream and sorbet scoops, rounded sides facing the plastic lining, alternating colors and keeping the scoops distinct. Repeat with more scoops until the inside of the bowl is completely lined with ice cream and sorbet. Return the bowl to the freezer and freeze until firm, 30 to 45 minutes.
Soften the remaining scoops slightly at room temperature, about 5 minutes. Place 1/2 cup of the cake cubes and 1/4 cup each of the marshmallow cereal and marshmallows in the bottom of the bowl. Fill with half of the softened scoops of ice cream and sorbet. Evenly spread another 1 cup cake and 1/4 cup each of the cereal and marshmallows over the ice cream and sorbet, then top with the remaining softened scoops, firmly pressing down to fill any holes. Top with the remaining cake, then wrap with the overhanging plastic wrap and freeze until set, about 1 hour.
To make the chocolate sauce, put the white chocolate and coconut oil in a microwave-safe bowl and heat, stirring every 30 seconds, until the chocolate is melted and smooth, 1 to 2 minutes.
Invert the bowl onto a serving platter, lift off the bowl and discard the plastic wrap. Drizzle the white chocolate sauce over the cake and immediately sprinkle with the sprinkles and remaining 1/2 cup cereal and marshmallows before the chocolate hardens. Decorate the bottom with a ring of cotton candy. Slice with a hot dry knife and serve immediately.
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