Spray three 6-inch round cake pans with nonstick spray, line with parchment paper and coat paper with nonstick cooking spray. Place 3 cups rice krispies in a large bowl. Place 3 remaining cups rice krispies in a separate large bowl and stir in jello powder. Place cocoa krispies in a third large bowl and stir in cocoa powder.
In a large pot, melt butter over medium low heat. Stir in marshmallows until melted and smooth. Working with one at a time, divide melted marshmallows into the three bowls, stirring to coat cereals completely, then transfer to prepared cake pans. Press gently to make even cakes and let set for at least 1 hour.
To assemble cakes, place the chocolate rice krispie cake on a plate or cake stand. Spread half of the buttercream frosting evenly over, then top with the white cake layer. Spread remaining buttercream frosting over and top with the strawberry krispie cake round.
Place chocolate chips is a medium bowl. In a small saucepan, heat heavy cream over medium high heat until small bubbles for around the edges and the cream is steaming. Pour over the chocolate and whisk to combine until smooth.
Use a spoon to make drips of chocolate down the edge of the cake, then fill in the center with remaining chocolate and use an offset spatula or knife to make a smooth and even frosting. Sprinkle 1/3 cup rainbow Jimmies over the top of the cake. Top with a large dollop of whipped cream in the center, then place an ice cream cone in the top of the whipped cream. Sprinkle with remaining jimmies and place a maraschino cherry in the whipped cream.