Line your 9×13-inch baking pan with foil or parchment paper, letting a little overhang on the sides for easy removal later. Spray the lining generously with cooking spray so nothing sticks once the mixture sets.
In a medium saucepan over medium-low heat, melt the butter first, then add the marshmallows. Stir constantly and keep the heat low, since high heat toward the end tends to make the bars turn out hard instead of soft.
Once the marshmallows are fully melted and smooth, remove the pan from the heat and stir in the vanilla extract. Quickly fold in the cereal so it gets evenly coated before the marshmallow mixture starts to firm up.
Fold in the M&Ms, candy corn, and sprinkles while the mixture is still warm and pliable. Work quickly but gently so the candy stays intact instead of breaking apart in the mix.
Pour everything into your prepared pan, then spray your hands with cooking spray and press the mixture down firmly and evenly. Let the pan cool completely at room temperature before slicing into squares.