Line an 8-inch square pan with foil so there is a 2-inch overhang on all sides. Lightly grease the foil with butter. Combine the blue, pink and purple sanding sugars together in a small bowl and set aside.
Melt the butter in a large pot over medium heat. Add the marshmallow chicks and cook, stirring, until completely melted, 5 to 6 minutes. Remove from the heat and stir in the vanilla and salt. Add both cereals and stir with a rubber spatula until combined. Transfer the mixture to the prepared pan. Coat the spatula in nonstick cooking spray and use it to press the mixture down into an even layer. Sprinkle the top with the sanding sugar mixture. Let sit at room temperature until firm, about 30 minutes. Cut into 12 bars. Store at room temperature in an airtight container for up to 3 days.
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