When your butter is melted, add 5 cups of Fruity Pebbles.
Reduce your heat to low and stir in half the bag of marshmallows until completely melted.
Remove from heat.
Stir in vanilla, then add the cereal and remaining marshmallows.
Line a 9 inch baking dish with parchment paper. (We like ours thin so we use a rectangle dish. For thick treats use a 9 inch square baking dish)
Pour in to the baking dish and press to the edges filling the dish. Press gently! The harder you press the harder they are to get out and eat. If you gently press they will be more airy than hard!