Spread the cake mix on a baking sheet and bake at 350°F for 5 minutes. Let cool completely.
Lightly spray a 9×9-inch pan with nonstick spray and set aside.
In a large pot over low heat, melt the butter and salt. Add 10 oz of the marshmallows and stir until completely melted and smooth.
Remove from the heat and stir in the vanilla and cooled cake mix until fully combined.
Fold in the Rice Krispies until evenly coated.
Stir in the remaining marshmallows and sprinkles.
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 it sit for 1 hour before cutting into squares.
Pipe the frosting on each square and add additional sprinkles. Enjoy!