Spray a 13x9 inch baking dish with non-stick spray. Set aside.
Add marshmallows and butter to a large mixing bowl and place in the microwave for 60 seconds. Remove and stir until completely melted. If necessary heat again for 20 seconds.
6 Tablespoons butter, 12 oz marshmallows
Add vanilla extract and mix until fully combined.
1 teaspoon vanilla extract
Pour in cereal, mix until the cereal is evenly coated.
6 cups Fruity Pebbles cereal
Press mixture into a sprayed 13x9 inch baking dish. Wet your hands (shake off any excess water) and press mixture evenly into the pan, or spray your spatula with non-stick spray.
Let bars set up for at least an hour before slicing.