Preheat the oven to 350 degrees. Spray a 9 x 13 baking dish with cooking spray and set aside.
In a large bowl combine the cookie mix, cake mix, eggs, butter, and vanilla. Mix until well combined.
Add the sprinkles and gently fold them in.
Pour the batter into the prepared pan and spread it out evenly.
Bake for 30-35 minutes or until the edges are golden brown and the center is puffy.
Allow to cool completely on a wire rack.
To make the frosting put the butter in a large bowl. Using a hand mixer, beat on medium speed until creamy.
Add in the powdered sugar, vanilla, and 2 tablespoons of milk. Mix on high for 1 minute. Add another tablespoon of milk if desired.
Spread the frosting over the cooled cookie bars. Sprinkle with the rainbow sprinkles.
Cut into 30 bars and serve.