Tool

• Three 8‑inch cake pans

• Parchment paper

• Cake strips

• Stand mixer with paddle attachment or handheld mixer

Ingredients

For the Cake

• 3¾ cups all‑purpose flour

• 2½ teaspoons baking powder

• 1 teaspoon salt

• 2⅓ cups granulated sugar

• 1 cup unsalted butter, softened

• ⅓ cup vegetable oil

• 1 tablespoon vanilla extract

• 1⅓ cups buttermilk, room temperature

• 7 large egg whites, room temperature

• ⅔ cup rainbow jimmies

For the Frosting

• 1 pound unsalted butter

• 2 pounds powdered sugar

• ½ teaspoon salt

• 1 tablespoon vanilla extract

• 3–4 tablespoons heavy cream

• Rainbow sprinkles, for serving

Direction

Make the Cake

1. Preheat oven to 350°F. Grease three 8‑inch pans, line with parchment, and use cake strips if desired.

2. Sift flour, baking powder, and salt into a large bowl; set aside.

3. In a stand mixer, beat sugar, butter, and oil on medium until smooth and creamy (about 3 minutes).

4. Add egg whites in three additions, mixing on low until combined. Scrape bowl as needed.

5. Add vanilla and beat until fluffy.

6. Add one‑third of the flour mixture; mix on low. Add half the buttermilk; mix. Repeat with another third of flour, remaining buttermilk, and final flour addition.

7. Fold in rainbow jimmies with a spatula.

8. Divide batter evenly among pans. Tap pans on a towel‑lined counter to release air bubbles.

9. Bake 32–38 minutes, rotating pans after 20 minutes, until centers spring back when gently pressed.

10. Cool cakes in pans for 10 minutes, then turn out onto wire racks and remove parchment.

Make the Frosting

1. Beat butter and salt on medium until very soft and smooth (about 2 minutes).

2. With mixer on low, gradually add powdered sugar, adding heavy cream 1 tablespoon at a time as needed.

3. Add vanilla. Increase speed to medium and beat 1–2 minutes until fluffy.

Assemble

1. Place one cooled cake layer top‑side up on a stand. Spread 1 cup frosting over the top.

2. Repeat with remaining layers.

3. Transfer 1 cup frosting to a piping bag fitted with a large open‑star tip.

4. Frost the outside of the cake.

5. Press rainbow sprinkles around the bottom 2–3 inches of the cake (“skirt”).

6. Pipe 8–10 dollops around the top edge and garnish with more sprinkles.

Notes

• No buttermilk? Mix 1⅓ cups whole milk + 1 tablespoon lemon juice or vinegar and let sit 5 minutes.

• You can swap in any frosting you prefer — vanilla, cream cheese, strawberry, chocolate, etc.

• For clean layers, wrap cakes and chill before assembly.

Nutrition

(per serving)

Calories: 1129kcal • Carbs: 154g • Protein: 7g • Fat: 55g
Saturated Fat: 32g • Polyunsaturated Fat: 5g • Monounsaturated Fat: 14g • Trans Fat: 2g
Cholesterol: 129mg • Sodium: 446mg • Potassium: 129mg • Fiber: 1g • Sugar: 123g
Vitamin A: 1516IU • Vitamin C: 0.02mg • Calcium: 104mg • Iron: 2mg