Line a 9 x 13 inch baking pan with aluminum foil. Liberally spray with non-stick cooking spray and set aside.
Add the butter, vanilla extract, and marshmallows to a large sauce pan. Over low heat, and stirring very often, melt the marshmallows until well incorporated into the melted butter.
Remove from heat. Add the cereal and half the sprinkles to the marshmallow mixture. Stir until well combined with a spatula.
Turn the mixture out onto the prepared baking dish. Press the mixture flat, trying to get the surface as even as possible. (If you find your hands are sticking to the mixture, lightly spray one palm with the non-stick cooking spray and rub your palms together.)
One the mixture has been flatten, sprinkle over the remaining sprinkles and press on them just lightly so that they stick to the surface.
Place in the freezer for 15 minutes. Remove from freezer and use your cookie cutter of choice to cut the shapes.
Can be stored in a food safe container in the fridge for up to five days.