Line a 9x13-inch baking pan with wax paper or parchment paper. Spray with cooking spray or butter the pan. Set aside.
Add the butter to a large saucepan or dutch oven over low heat. Stir until the butter is melted.
Add all but 2 cups of the mini marshmallows to the butter and melt over low heat, stirring occasionally. It will take about 5-6 minutes for the mini marshmallows to melt. Do not raise the heat as this will make the marshmallows tough.
Add the vanilla extract and stir to combine.
Take the pot off the heat. Add the remaining 2 cups of mini marshmallows. Stir for 1-2 minutes, until the mini marshmallows are partially melted. You do not want them to melt all the way.
Pour the gluten-free crispy rice cereal into the pot. Stir until the cereal is covered completely with the melted marshmallows.
Add the confetti sprinkles to the rice krispies, reserving about 1 teaspoon for later. Stir just until the sprinkles are distributed.
Spoon the rice krispies into the prepared pan. With a greased spatula, lightly press the cereal mixture down until even.
Top with remaining confetti sprinkles. Let cool for an hour before slicing and serving.