Ingredients

For the Vanilla Sheet Cake:

• 2 & 1/2 cups (265 g) sifted cake flour
• 2 tsp baking powder
• 1/2 tsp baking soda
• 1 tsp salt
• 3/4 cup (170 g) unsalted butter, softened
• 1 & 3/4 cups (340 g) granulated sugar
• 3 large eggs, room temperature
• 1/2 cup (120 g) sour cream, room temperature
• 1 tbsp pure vanilla extract
• 1 cup (240 ml) whole milk, room temperature

For the Buttercream Frosting:

• 2 cups (4 sticks) unsalted butter, softened
• 8 ounces cream cheese, softened (optional)
• 1/2 tsp kosher salt
• 8 cups powdered sugar
• 2 tsp vanilla extract
• 4 to 5 tbsp heavy cream, half-and-half, or milk

For Decorating:

• Dark blue gel food coloring
• Medium blue gel food coloring
• Light blue gel food coloring
• Red gel food coloring or pre-colored red buttercream
• White buttercream reserved from the frosting

Instructions

Preheat the oven to 325°F. Spray a 9 x 13-inch metal baking pan with baking spray and line the bottom with parchment paper.

Make the Cake:

In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high speed until smooth and creamy, about 2 minutes. Add the sugar and continue beating until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Mix in the vanilla extract and sour cream until fully incorporated. Add the dry ingredients and mix on low speed just until combined. Slowly pour in the milk and mix until incorporated. Scrape down the bowl and stir by hand to ensure the batter is smooth.

Spread the batter evenly into the prepared pan. Bake for 30 to 35 minutes, or until the cake springs back when lightly touched and a toothpick inserted into the center comes out clean.

Allow the cake to cool completely in the pan before frosting.

Make the Buttercream:

Beat the butter until completely smooth and creamy. If using cream cheese, add it now and continue beating until fully incorporated. Mix in the salt.

Add 4 cups of powdered sugar, 1 cup at a time, mixing well after each addition. Add the vanilla extract and 2 tablespoons cream. Continue adding the remaining powdered sugar, mixing after each addition. Add additional cream as needed until the frosting reaches your desired consistency. Beat for an additional minute until light and fluffy.

Frost the Cake:

Spread a thin, even layer of buttercream over the cooled cake. Refrigerate for 20 minutes.

Divide the remaining frosting among separate bowls and tint dark blue, medium blue, light blue, and red, leaving some frosting white.

Decorate:

Create a rectangle approximately 5 & 1/2 inches by 4 & 1/2 inches in the upper left corner of the cake and fill it with dark blue frosting.

Use a measuring tape and toothpick to lightly mark 1/2-inch stripes across the cake. Fill every other stripe with red frosting, leaving the remaining stripes white or pipe white stars into the white stripes.

Pipe rosettes and flowers over the blue section using a large round tip and star tips. Fill any open spaces with smaller flowers or stars.

Chef’s Notes:

• Homemade cake flour can be made by removing 2 tbsp from each cup of all-purpose flour and replacing it with 2 tbsp cornstarch. Sift the mixture 4 times before measuring.
• Pre-colored red buttercream is often easier than tinting your own frosting, as achieving a deep red color typically requires a large amount of food coloring.
• If using a glass or ceramic baking dish, increase the baking time by 10 to 15 minutes.
• The unfrosted cake can be wrapped tightly and stored at room temperature for 2 days or frozen for up to 2 months.
• Buttercream can be refrigerated for up to 2 weeks. Bring to room temperature and rewhip before using.
• The finished cake can be covered and stored at room temperature for a day or in the refrigerator for 5 days.