Prepare a 9x13 baking dish by greasing it with cooking spray and lining it with parchment paper. Set aside.
Add the butter to a large saucepan and melt it completely over the stove, using low-medium heat.
Add the vanilla extract and the marshmallows (9 cups), reserving 1 cup for later, and stirring continuously until all of the marshmallows have melted.
Remove from heat and immediately add the Rice Krispies, then the candy corn and reserved marshmallows. Stir until everything is covered with marshmallows.
Immediately transfer to the prepared baking dish and use a greased spatula, greased parchment paper piece or grease your hand to press the bars evenly into the pan.
Allow the bars to set for at least 20 minutes before enjoying them. Store at room temperature.
