Preheat oven to 350°F and grease bottom of 13x9-inch pan with cooking spray.
In large bowl, beat sugar cookie mix, cake mix, melted butter, eggs and 1 teaspoon vanilla with electric mixer on low speed 30 seconds. Stir in sprinkles and spread evenly across the bottom of pan.
Bake 32 to 34 minutes or until golden brown.
Cool completely.In large bowl, beat 1 cup softened butter, the powdered sugar, 2 tablespoons milk and 1 teaspoon vanilla until combined.
Add additional milk 1 teaspoon at a time until spreading consistency. Spread over cooled bars.
Sprinkle with assorted candy sprinkles. Cut into small pieces and serve!