Spread the sugar cookie mix on a baking sheet and bake at 350°F for 5 minutes. Let cool completely.
Lightly spray a 9×13-inch pan with nonstick spray and set aside.
In a large pot over low heat, melt the butter and salt. Add 14 oz of the marshmallows and stir until completely melted and smooth.
Remove from the heat and stir in the vanilla and cooled cookie mix until fully combined.
Fold in the Rice Krispies until evenly coated.
Let the mixture cool for 1 minute, add the remaining 2 oz marshmallows, and gently fold in the 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.