Spray a 9 x 9-inch pan with cooking spray then line with parchment paper, leaving an overhang for easy removal. Separate 2-1/2 cups (120 grams) of marshmallows from the bag.
In a large nonstick pot, melt the butter over low heat. Once the butter is melted, add all but the reserved 2-1/2 cups (120 grams) of the marshmallows. Stir constantly over low heat until the marshmallows are completely melted.
Remove the pot from the heat and stir in the salt and vanilla extract. Gently fold in the Rice Krispies and Fruity Pebbles cereals. Add the reserved 2-1/2 cups of marshmallows and stir until everything is well combined. Take your time here, and don't mash the cereal while stirring.
Pour the mixture into the prepared pan. Lightly spray your hands with cooking spray and gently press the mixture into an even layer. Be careful, pressing too hard can make the bars dense and heavy.
Allow the bars to cool and set for at least an hour. Use the parchment paper overhang to lift the bars out of the pan and cut them into squares with a sharp knife.
Video
Notes
Note 1: The softer and fresher the marshmallows, the better these bars will taste.Note 2: You can use all Fruity Pebbles instead of the Rice Krispies and Fruity Pebbles mix, but I found it to be a little too sweet. Storage: For the best texture, enjoy these the same day. Store leftovers tightly wrapped at room temp for up to 2 days.