Prepare and bake cake according to package directions, using a 13×9-in. cake pan. Let cool. Using a meat fork or the handle of a wooden spoon, pokes holes in the cake every 1/2-in to 1-in.
Empty the jello packages into a medium-sized bowl and add the boiling water. Stir briskly for 2 minutes or until the jello is dissolved. Evenly pour over the poked cake. Place in the refrigerator for 2-4 hours or until jello is set.
Meanwhile, make the frosting. Cream butter and the vanilla with an electric mixer. Add the powdered sugar and 3 tbsp. milk. Beat with an electric mixer until well combined and fluffy. Add a small amount of blue food coloring, beat well. Add more food coloring if more color is desired. If necessary, add 1 tbsp. milk to the frosting if it needs thinned out.
After jello is set, top cake with frosting. Top with sprinkles.