Line a 9-by-13-inch baking dish with foil, leaving a 2-inch overhang on at least two sides; butter the foil. Pick out and reserve 1/2 cup the colorful marshmallows from the cereal.
Melt the butter in a large saucepan over medium-low heat. Add the standard marshmallows and stir continuously until melted, about 5 minutes. Remove from the heat and stir in the cereal until coated.
Press the mixture into the prepared baking dish and let cool completely at room temperature, about 1 hour.
Meanwhile, put the chocolate in a medium microwave-safe bowl. Microwave for 30 second increments, stirring after each, until melted.
Use the foil overhang to lift out the treats from the pan. Drizzle the chocolate over the treats and sprinkle with the reserved colorful marshmallows. Let the chocolate coating harden at room temperature, about 2 hours. Cut into 18 bars.
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