This rainbow sprinkle cake from Beyond Frosting is an over-the-top dessert. While this cake utilizes boxed mix, it doesn't make it any less delicious. Once baked, the cake is pierced with holes and soaked in sweetened condensed milk. To make this cake even more decadent, it is topped with cake batter flavored whipped cream and Birthday Cake Oreos. To bake this delicious rainbow sprinkle birthday cake yourself, click here.