Butter a 9 x 13 dish, set aside. Melt butter in a large nonstick saucepan over medium low heat. Stir in salt. Add marshmallows and stir constantly until completely melted. Remove from heat. Stir in Sugar Cookie Mix. Fold in Rice Krispies Cereal and stir until evenly coated. Pour mixture into buttered dish. Using a buttered spatula, spread mixture evenly into pan. Allow to cool slightly, about 2 minutes, then cut into 12 squares. Using your hands, round edges of squares by cupping your hands and gently pressing on corners (I also flattened them a bit but not necessary). Frost each "cookie" with Sugar Cookie Frosting and sprinkle with optional sprinkles. Store in an airtight container.
Sugar Cookie Frosting
In a mixing bowl, using an electric mixer, whip together butter, milk, vanilla, salt and powdered sugar until fluffy and smooth about one minute. Mix in several drops optional food coloring, stir until well blended.