Preheat oven to 350 degrees F. and spray four 4-6-inch cake pans generously with cooking spray.
Place cake mix, eggs, milk, oil, and sprinkles into a large mixing bowl. Stir until well combined then pour into prepared cake pans, spreading evenly. Bake for 25-35 minutes or until toothpick comes clean when placed in centers of cake. Remove and let cool completely.
Place condensed milk and peanut butter into a small bowl, mixing until well combined.
The four cakes will make 2 individual birthday cakes. Place 2 cakes onto separate serving plates. Spread peanut butter layer on top of each. Top with 2nd cooked cake. drizzle melted chocolate over each other cakes. Arrange how you'd like the candy bars, pixie sticks, suckers, ring pops and cookies. See photos for any help. Let chocolate set up and serve room temperature.