Step 1

 Grease a 9-by-13-inch baking pan with cooking spray.

Step 2

Melt the butter in a large stock pot over medium-low heat. Carefully stir in the vanilla extract (it will spatter), then add the marshmallows and cook, stirring, until smooth, about 5 minutes. Stir in the popcorn and 1/4 cup of the sprinkles until the popcorn is well coated, then immediately pour the mixture into the prepared baking dish, pressing it firmly into an even layer. Sprinkle the remaining 1/4 cup sprinkles on top.  

Step 3

Let the treats sit for 20 minutes then slice into squares or bars and serve.

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