Ingredients

For the Rainbow Cake Layers

  • 2 cups (240g) cake flour
  • 1 ½ cups (300g) granulated sugar
  • 2 tsp baking powder
  • ½ tsp baking soda
  • ½ tsp kosher salt
  • ½ cup (115g) unsalted butter, softened
  • ¼ cup (60ml) vegetable oil
  • 3 large eggs
  • 1 cup (240ml) buttermilk
  • 2 tsp vanilla extract
  • Gel food coloring (red, orange, yellow, green, blue, purple)
  • Cooking spray or butter for greasing pans

For the Milk Crumb

  • ½ cup (40g) milk powder
  • ¼ cup (30g) all-purpose flour
  • 2 tbsp cornstarch
  • 2 tbsp granulated sugar
  • ¼ tsp kosher salt
  • ¼ cup (55g) unsalted butter, melted
  • ¼ cup (20g) milk powder (for tossing)
  • 3 oz (85g) white chocolate, melted

For the Vanilla Frosting

  • 1 cup (225g) unsalted butter, softened
  • 4 cups (480g) powdered sugar, sifted
  • ¼ cup (60ml) heavy cream
  • 2 tsp vanilla extract
  • ¼ tsp kosher salt
  • Rainbow sprinkles (for decoration)

For the Milk Soak

  • ¼ cup (60ml) whole milk
  • 1 tsp vanilla extract

Equipment

  • 6-inch round cake pans (3 or more, or bake in batches)
  • Parchment paper
  • Mixing bowls
  • Electric mixer or stand mixer
  • Cooling racks
  • 6-inch cake ring (optional, for assembly)
  • Acetate strips (for assembly, optional)
  • Offset spatula

Instructions

Step 1: Prepare the Milk Crumb

  1. Preheat oven to 250°F (120°C). Line a baking sheet with parchment paper.
  2. In a medium bowl, combine ½ cup milk powder, flour, cornstarch, sugar, and salt.
  3. Add melted butter and stir until mixture forms small clusters.
  4. Spread clusters on the prepared baking sheet and bake for 20 minutes, until lightly golden. Let cool completely.
  5. Break up clusters and toss with remaining ¼ cup milk powder. Drizzle melted white chocolate over crumbs and toss until coated. Let set in the fridge for 10 minutes.

Step 2: Bake the Rainbow Cake Layers

  1. Preheat oven to 350°F (175°C). Grease and line three 6-inch cake pans with parchment paper.
  2. In a large bowl, whisk together cake flour, sugar, baking powder, baking soda, and salt.
  3. In a separate bowl, beat butter and oil until smooth. Add eggs one at a time, then vanilla extract.
  4. Gradually add dry ingredients to wet, alternating with buttermilk, starting and ending with dry. Mix until just combined.
  5. Divide batter evenly into six bowls (about ¾ cup per bowl). Add a different gel food coloring to each bowl (red, orange, yellow, green, blue, purple) and mix until vibrant.
  6. Pour three colors into separate prepared pans (bake in batches if you have fewer pans). Bake for 18-22 minutes, or until a toothpick inserted comes out clean. Let cakes cool for 10 minutes in pans, then transfer to cooling racks. Repeat for remaining colors.

Step 3: Make the Vanilla Frosting

  1. In a large bowl, beat softened butter until creamy using an electric mixer.
  2. Gradually add powdered sugar, 1 cup at a time, beating on low until combined.
  3. Add heavy cream, vanilla extract, and salt. Beat on high for 2-3 minutes until light and fluffy. Set aside.

Step 4: Prepare the Milk Soak

  1. In a small bowl, mix milk and vanilla extract. Set aside.

Step 5: Assemble the Cake

  1. If using a cake ring, place it on a cake board or flat surface. Line with an acetate strip.
  2. Place the purple cake layer in the ring (or on a cake board if not using a ring). Brush with 1-2 tbsp of milk soak.
  3. Spread a thin layer of frosting over the cake, then sprinkle with milk crumb.
  4. Add the blue layer, press gently, and repeat with milk soak, frosting, and crumbs. Continue with green, yellow, orange, and red layers, repeating the process.
  5. Apply a thin “crumb coat” of frosting over the top and sides of the cake. Chill for 30 minutes.
  6. Spread a final thick layer of frosting over the top and sides, smoothing with an offset spatula. Decorate with rainbow sprinkles.
  7. If using acetate, chill the cake for 1 hour, then remove the ring and acetate.

Step 6: Serve

  1. Slice and serve at room temperature for best flavor. Store leftovers in an airtight container in the fridge for up to 3 days.

Tips

  • Use gel food coloring for vibrant colors; liquid food coloring may dilute the batter.
  • If you don’t have multiple cake pans, bake in batches and clean pans between uses.
  • For a cleaner look, use a cake ring and acetate strips, but a standard cake board works fine.
  • Customize sprinkles or add edible glitter for extra flair.

Nutritional Information (Per Serving, Approx.)

  • Calories: 520
  • Fat: 28g
  • Carbohydrates: 65g
  • Protein: 5g

Enjoy your colorful, delicious Rainbow Milk Bar Cake!