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Generously grease a 9×13″ cake pan or baking dish with butter.
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Melt one stick of butter in a large pot over medium heat.
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Add mini marshmallows and begin stirring with a heavy-duty spatula. Then add 2/3 of the white chocolate chips and continue stirring.
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Continue stirring until marshmallows are completely melted. There may still be some small chunks of white chocolate–that’s okay..
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Slowly add the Rice Krispies to the pot and stir until cereal is completely coated with marshmallow mixture.
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Pour mixture (it will be sticky!) into your pre-greased baking dish.
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Butter your hands, then press down cereal into pan.
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Melt remaining white chocolate chips in microwave. (Heat for 30 seconds, then stir, then repeat until melted. If mixture seems thick, add a tablespoon of vegetable oil.)
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Place melted white chocolate in a pastry bag (or Ziploc bag, in a pinch). Cut off tip (or corner) of bag, then drizzle over-top of cereal, then sprinkle with rainbow sprinkles. Refrigerate for at least 30 minutes to let white chocolate harden before attempting to cut into squares.