In a medium saucepan over low heat, melt the 2 cups white melting chocolate and 1/2 cup salted butter, stirring until smooth. Remove as soon as it melts.
Pour the mixture over the 8 cups Rice Chex cerealin a large bowl. Toss gently to cover evenly without breaking the cereal.
Combine the 8.25 ounces funfetti cake mix*, 1 cup powdered sugar and 2-3 tablespoons rainbow sprinkles in a large plastic bag.
Add half of the tacky cereal into the bag, shaking until covered. Using your hands or a slotted spoon, remove it to a parchment lined baking sheet.
Repeat with the rest of the cereal.
Allow to set at room temperature in the fridge.
Store in an airtight container at room temperature or in the fridge.
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Notes
Raw cake mix or flour can be unsafe. Cake mix can be heated treated, but funfetti sprinkles will melt. If you choose to heat treat, use a plain white cake mix and add 1/2 cup of rainbow sprinkles after it cools.