Melt the butter in a large pot. Add 3 1/2 cups of mini marshmallows and stir until melted. Add vanilla extract. Take off the heat.
Add in the cereals and 1/2 cup of mini marshmallows, and stir until combined.
Put the mixture into a greased 8×12 pan. Spray a spatula with cooking spray and use the greased spatula to spread and flatten the mixture. Sprinkle the sprinkles on top.