Special equipment: a 17-by-11-inch jelly roll pan; 7 piping bags fitted with a 1/2-inch plain decorator tip; a 14-by18-inch cake board or large platter
Print the OMG Letter Templates (link below) and cut them out with scissors. Place the letters on one of the sheets of Sponge Cake and use a sharp knife or kitchen-dedicated X-Acto knife to cut out the letters. Repeat the process with the remaining cakes so you have 3 of each letter. Discard the cake scraps or save for ice cream sundae toppings.
Place one set of letters on a 14-by-18-inch cake board or large platter, spacing them about 1 inch apart. Pipe mounds of the Buttercream Frosting in alternating colors onto the cake pieces. Stack one set of matching letters on top of the first set of letters. Pipe the second layer of cake pieces with more mounds of buttercream. Add the final layer of cake pieces on top and pipe mounds of buttercream on top. Add the rainbow nonpareils and confetti sprinkles immediately.
Decorate the top of the cake with the mini chocolate bars, donuts, rock candy lollipops, gummy candies and chocolate balls. Serve at room temperature. Store leftovers covered in plastic wrap in the refrigerator.
Preheat the oven to 350 degrees F. Spray a 17-by-11-inch jelly roll pan with flour-based baking spray.
Place the eggs in a stand mixer fitted with the whisk attachment and, with a timer set, beat for 5 minutes. Slowly add the granulated sugar and oil. Beat well. Add the milk and vanilla. Switch to the paddle attachment. In a separate bowl, whisk together the flour, baking powder and salt. Add slowly to the liquid ingredients. Beat for 2 minutes, until well combined. The batter will be thin.
Pour the batter into the prepared jelly roll pan and bake for 10 minutes. When done, the cake should spring back when touched in the center. Immediately turn it out onto a wire rack to cool completely.
For the buttercream frosting: In a stand mixer fitted with the whisk attachment, add the butter and confectioners' sugar. Begin mixing on low speed until the mixture is crumbly. Increase to high and beat for 3 minutes.
Add the vanilla extract and beat again. While mixing, add cream or milk 1 tablespoon at a time until the mixture comes to piping consistency.
Divide the frosting evenly into 7 separate bowls. Tint each bowl of frosting with one of the 6 colors (pink, orange, yellow, green, blue and violet), leaving one bowl untinted; begin with 1 teaspoon of food coloring and add more to intensify the color if needed. Transfer each color of frosting to a separate piping bag fitted with a 1/2-inch plain decorator tip.