Lightly spray a 9×13-inch pan and set aside.
In a large pot over medium-low heat, melt the butter and stir in the salt.
Add the sweetened condensed milk and stir until smooth and fully combined.
Add 14 oz of the mini marshmallows and stir gently until completely melted and smooth. Remove from the heat immediately once melted.
Stir in the vanilla extract.
Add the Fruity Pebbles cereal and gently fold until evenly coated.
Fold in the remaining 2 oz mini marshmallows to create soft, gooey marshmallow pockets throughout the treats.
Transfer the mixture to the prepared pan. Use lightly buttered or damp hands to gently press the treats into an even layer. Don’t compact them or the bars will turn dense.
Let the treats cool at room temperature for 1 hour before slicing into squares.