Melt the butter in a sauce pan on medium heat. Once melted, add in the marshmallows and heat on low until melted. I always save a couple of marshmallows to mix in toward the end, so feel free to save a few (I'll explain soon!). It is important to keep the heat on low or your marshmallow might harden too quickly. Once the marshmallows have melted completely, remove from heat, and add in the vanilla and salt. Then, mix in the Fruity Pebbles. If you saved any marshmallows, break them into little pieces and stir them in at this point.