Make the cake: Position a rack in the lower third of the oven and preheat to 350 degrees F. Butter four 9-inch-round pans and line the bottoms with parchment paper. Combine the granulated sugar and vegetable oil in a large bowl and beat with a mixer on medium-high speed until thick and creamy, about 3 minutes. Beat in the eggs, one at a time, then add the vanilla and beat on high speed until the batter is light and fluffy, about 2 minutes.
Sift the flour, baking soda and salt into a medium bowl. With the mixer on low speed, beat the flour mixture into the sugar mixture in 3 additions, alternating with the buttermilk, beginning and ending with the flour mixture, until the batter is smooth, about 2 minutes. Tint with gel food coloring (see below).
Divide the batter among the prepared pans and bake, 2 pans at a time, until a toothpick inserted into the centers comes out clean, about 15 minutes. Let cool 15 minutes in the pans, then invert onto racks to cool completely.
Make the frosting: Beat the butter and salt in a large bowl with a mixer on medium speed until smooth and fluffy, about 2 minutes. Gradually beat in the confectioners' sugar until smooth. Add the vanilla, increase the mixer speed to medium high and beat until thick, 2 more minutes. Beat in the milk, a little at a time, until the frosting is spreadable.
Assemble the cake: Spread about 1 1/2 cups frosting between the cake layers, then cover the outside of the cake with the remaining frosting and coat with toppings (see below).
How to Color the Batter:
Use gel food coloring for cakes: You don't need to add as much, and the colors are brighter.
Solid: Tint the batter 1 color; divide among 4 pans.
Striped: Divide the batter between 2 bowls; tint each one a different color. Divide each color between 2 pans.
Ombre: Divide the batter among 4 bowls; tint in varying shades of the same color. Pour into 4 pans.
Rainbow: Divide the batter among 4 bowls; tint each one a different color. Pour into 4 pans.
Mini Marshmallows: You'll need about one 10 1/2-ounce bag. For a multicolored look, use fruit-flavored mini marshmallows.
Chocolate Rocks: You'll need about 3 cups. Find them at specialty candy stores, or order online.
Ombre Coconut: Divide one 7-ounce bag coconut among 3 resealable plastic bags. Add an increasing number of drops of the same food coloring to each bag and shake.
Nonpareils: Use rainbow or single-color nonpareils; you'll need about 3/4 cup. Set your cake on a cake plate in the sink or on a large rimmed baking sheet to catch loose nonpareils.
Photograph by Justin Walker
Food Network Magazine made the cake layers in four separate pans: They bake quickly and they come out even, so there's no need to trim the tops. If you only have two pans, bake the layers in two batches, allowing the pans to cool in between.
Courtesy of Food Network Magazine