Stir together 1 teaspoon cornstarch and 1 Tablespoon water in a medium mixing bowl until cornstarch is full dissolved.
Add your frosting to the bowl and stir to combine.
Add the rest of your cornstarch little by little until the mixture clumps together to form a batter.
Finish kneading by hand.
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Notes
While this birthday cake play dough recipe is edible, it is not intended to be consumed in large amounts. Always supervise children when making and playing.