1-First Step: Prepare the vanilla cake batter. In a large bowl, beat 1 cup softened unsalted butter and 2 cups granulated sugar until light and fluffy, about 3 minutes. Add 4 large eggs one at a time, beating well after each. Mix in 1 tablespoon vanilla extract. In another bowl, whisk 3 cups all-purpose flour, 1 tablespoon baking powder, and 1/2 teaspoon salt. Alternate adding dry ingredients and 1 1/2 cups whole milk to the butter mixture, starting and ending with flour. Beat until just combined, avoiding overmixing for tender layers. This base works for vegan swaps too, like plant milk.

2-Second Step: Divide and color the batter. Portion batter into six bowls, about 1 cup each. Add 1/4 to 1/2 teaspoon gel food coloring per bowl: red to first, mix red and yellow for orange in second, yellow to third, mix yellow and blue for green in fourth, blue to fifth, mix red and blue for purple in sixth. Stir gently until uniform. For vibrant results without fading, chill tinted batter 10 minutes. This step shines in the easy rainbow cake recipe.

3-Third Step: Bake the colorful layers. Spread each batter evenly into prepared pans. Bake at 350°F for 10 to 12 minutes, until toothpick inserted comes out clean. Rotate pans halfway if oven heats unevenly. Cool in pans 5 minutes, then transfer to wire racks to cool completely, about 30 minutes. Thin baking prevents doming and keeps layers moist. Gluten-free batter may need 1 extra minute.

4-Fourth Step: Make the buttercream frosting. Beat 2 cups softened unsalted butter until creamy. Gradually add 8 cups powdered sugar, 1/4 cup milk, and 2 teaspoons vanilla, beating until light and spreadable. For low-calorie, use half the sugar and more milk. Whip 3 to 5 minutes for fluffiness. Reserve 2 cups plain white; tint rest lightly if desired, but white highlights rainbows best.

5-Fifth Step: Assemble the layer cake. Level cooled layers if needed. Place purple layer on cake board, spread 1/2 cup frosting evenly. Stack blue, green, yellow, orange, red layers, frosting between each. Apply thin crumb coat with remaining frosting, chill 30 minutes to set. Frost exterior smoothly. For stability, use a turntable.

6-Final Step: Finishing touches and serving. Chill assembled cake 30 minutes more. Slice to reveal rainbow layers. Serve at room temperature for best texture. Pairs great with fresh berries or ice cream. Store as noted below. This rainbow cake for birthdays wows every time.