Lightly spray an 8×8 inch pan. Set aside.
In a large pot over low heat, melt the butter and salt. Add 10 oz of the marshmallows and stir gently until just melted and smooth.
Remove from heat and stir in vanilla.
Add the Froot Loops and fold until everything is 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.