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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.
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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.
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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.
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4
Sift the flour, baking soda and salt into a medium bowl.
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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.
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Tint with gel food coloring (see below).
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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.
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Let cool 15 minutes in the pans, then invert onto racks to cool completely.
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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.
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Gradually beat in the confectioners' sugar until smooth.
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Add the vanilla, increase the mixer speed to medium high and beat until thick, 2 more minutes.
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Beat in the milk, a little at a time, until the frosting is spreadable.
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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).
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How to Color the Batter:
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Use gel food coloring for cakes: You don't need to add as much, and the colors are brighter.
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Solid: Tint the batter 1 color; divide among 4 pans.
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Striped: Divide the batter between 2 bowls; tint each one a different color.
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Divide each color between 2 pans.
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Ombre: Divide the batter among 4 bowls; tint in varying shades of the same color.
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Pour into 4 pans.
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Rainbow: Divide the batter among 4 bowls; tint each one a different color.
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Pour into 4 pans.
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The Toppings:
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Mini Marshmallows: You'll need about one 10 1/2-ounce bag.
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For a multicolored look, use fruit-flavored mini marshmallows.
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Chocolate Rocks: You'll need about 3 cups.
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Find them at specialty candy stores, or order online.
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Ombre Coconut: Divide one 7-ounce bag coconut among 3 resealable plastic bags.
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Add an increasing number of drops of the same food coloring to each bag and shake.
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Nonpareils: Use rainbow or single-color nonpareils; you'll need about 3/4 cup.
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Set your cake on a cake plate in the sink or on a large rimmed baking sheet to catch loose nonpareils.
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Photograph by Justin Walker
