Line a 8x8 or 9x9 with parchment paper. Lightly grease the parchment. I always use a very light mist of nonstick spray. Set aside.
Melt the butter over low heat in a large pot or Dutch oven. Once melted, add the 10-ounce bag of marshmallows. Stir the mixture until the marshmallows are just melted. Stir in the salt and vanilla to combine.
Remove the pan from the heat, add the Fruity Pebbles™ cereal, and mix until evenly coated with the marshmallow mixture. Stir in the additional one cup of marshmallows.
Turn the mixture out into the prepared pan and, using a wooden spoon or spatula that has been coated in non-stick cooking spray, press the mixture into a packed, even layer. Do not pack it down with force. Lightly press down until it is secure in the pan.
Allow treats to set for at least 1 hour at room temperature and up to 1 day. Cover tightly if leaving out for more than a few hours.
Lift the treats as a whole out of the pan using the edges of the parchment paper. Cut into squares.
Cover and store leftover treats at room temperature for up to 3 days. To store, place in an airtight container, with sheets of parchment or wax paper between layers.