We use a triple dose of cereal — in the crust, steeped in milk for the filling and on top to decorate — in this over-the-top rainbow cake.
Place the milk and 1 cup of the cereal in a medium bowl and let sit until the milk takes on the cereal flavour, about 30 minutes. Strain the milk and discard the cereal. Set the milk aside.
Meanwhile, melt the butter in a medium saucepan over medium-low heat. Add the marshmallows and stir continuously until melted, about 5 minutes. Remove from the heat and stir in the remaining 3 cups cereal until coated.
Press the mixture into a 9-inch springform pan and let cool completely at room temperature, about 30 minutes.
Combine the gelatin with 1/4 cup water in a small microwave-safe bowl and set aside to soften, about 5 minutes. Beat the cream cheese, confectioners’ sugar, sour cream, lemon juice and vanilla together in a large bowl with an electric mixer on medium-high speed until completely smooth, about 1 minute. Scrape down the sides of the bowl and add the cereal milk. Continue to beat on low until completely incorporated, about 1 minute.
Microwave the gelatin, stirring as needed, until melted, about 30 seconds. Add the gelatin to the cream cheese mixture and beat on medium speed to combine, about 30 seconds. Pour the cream cheese mixture on top of the cereal crust and smooth out the top if needed. Refrigerate until completely set, at least 4 hours and up to overnight.
Evenly divide the white chocolate among 3 small bowls. Heat the heavy cream in a small saucepan until boiling. Remove 2 tablespoons of the cream to a small bowl and colour with the yellow food colouring, then pour over the chocolate in one of the bowls. Colour another 2 tablespoons of the cream with the blue food colouring in the same manner and pour over the chocolate in another bowl. Colour the remaining 2 tablespoons cream with pink food colouring and pour over the last bowl of chocolate. Let all three bowls of chocolate sit for 5 minutes, then stir until completely smooth.
Open the clasp of the springform pan to unmold the cheesecake. Drizzle the coloured chocolate in alternating sections around the edge. Sprinkle more cereal on top of the cake in a ring inside the coloured-chocolate edging. Pipe or dollop whipped cream inside the cereal circle and top each dollop with a Lucky Charms® marshmallow.