Line a 9x13 pan with parchment paper and spray with cooking spray. Set aside.

Melt butter in a large Dutch oven or pot. Add two bags of marshmallows and stir until completely melted. Remove from heat and add salt and vanilla.

Add Rice Krispies, Fruity Pebbles, and the additional bag of marshmallows to melted marshmallow. Stir to coat completely.

Turn out into prepared pan. Spray an offset spatula with cooking spray and press into pan. Let set in pan for one hour before cutting into bars.