Grease a loaf pan with butter or baking spray. Line the pan with parchment paper and set aside for later.

Then, melt the butter over medium-low heat. Add the mini marshmallows and stir constantly, just until they're melted and well-combined with the butter. Stir in the salt and vanilla extract.

Remove the pan from the heat and add the Fruity Pebbles cereal. Stir gently, but quickly, until everything is well-combined. Then, fold another 1-2 handfuls of mini marshmallows into the mixture if you want extra gooey bars.

Using a greased rubber spatula, gently press the mixture evenly into the prepared loaf pan. Let the bars set at room temperature or in the fridge for 30-60 minutes. Use the parchment paper to remove the treats from the pan. Slice into 6-8 bars and enjoy!