In a large bowl, combine the marshmallows and butter. Microwave for 2 minutes, stirring after 1 minute. Add 5 cups of cereal and stir until full mixed. Spoon into the bottom of a greased 8-inch springform pan, pressing down around the edges to create a smooth surface. Transfer to the refrigerator to chill for about 20 minutes.
In a large bowl, whisk the cream cheese and lemon juice until combined. Add the cream, vanilla, and sugar and mix until smooth and silky with soft peaks.
Pour the cream cheese mixture on top of the rice cereal and marshmallow base, using a spatula to smooth the top. Chill for at least 3 hours.
Release the springform and decorate the top of the cake with the remaining cup of rice cereal. Slice and serve.