A simple American buttercream improves a flag sheet cake

BY AMERICA'S TEST KITCHEN

Published: Wed, July 4, 2018 5:00 AM

U.S. flag cake appears in the cookbook “The Perfect Cake.” [Photo by Carl Tremblay, America's Test Kitchen/AP]

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A big flag sheet cake is the perfect Fourth of July dessert; and at a time when patriotic-colored berries are at their best, vibrant blueberries and red raspberries are a fitting topping.

We knew that our workhorse Yellow Sheet Cake was the ideal base for the cake, so our challenge when developing this recipe was simply determining the best frosting.

Whipped cream, the most common topping for this summer cake, had little staying power and the juicy berries bled into the cream. A simple American buttercream in the form of our vanilla frosting worked much better and ably glued the stars and stripes to the cake.

Once our cake was topped with berries, we were able to bring the flag to the table. You may have extra frosting after coating the tops and sides of the cake. Wait until a few hours before serving to arrange the berries to ensure that they taste fresh at serving time.

FLAG CAKE

Servings: 12-15

Start to finish: 1 hour and 30 minutes, plus 2 hours to cool the cake

YELLOW SHEET CAKE:

2 1/2 cups (10 ounces) cake flour

1 3/4 cups (12 1/4 ounces) sugar

1 1/4 teaspoons baking powder

1/4 teaspoon baking soda

3/4 teaspoon salt

1 cup buttermilk, room temperature

3 large eggs, separated, plus 3 large yolks, room temperature

10 tablespoons unsalted butter, melted and cooled

3 tablespoons vegetable oil

2 teaspoons vanilla extract

Pinch cream of tartar

VANILLA FROSTING (5 CUPS):

1 pound (4 sticks) unsalted butter, each stick cut into quarters and softened

1/4 cup heavy cream

1 tablespoon vanilla extract

1/4 teaspoon salt

4 cups (16 ounces) confectioners' sugar

TOPPINGS:

5 ounces (1 cup) blueberries

15 ounces (3 cups) raspberries

Adjust oven rack to middle position and heat oven to 325 F. Grease and flour 13 by 9-inch baking pan. Whisk flour, 1 1/2 cups sugar, baking powder, baking soda and salt together in bowl. Whisk buttermilk, egg yolks, melted butter, oil and vanilla together in second bowl.

Whisk flour, 1 1/2 cups sugar, baking powder, baking soda and salt together in bowl. Whisk buttermilk, egg yolks, melted butter, oil and vanilla together in second bowl.

Using stand mixer fitted with whisk attachment, whip egg whites and cream of tartar on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and whip whites to soft billowy mounds, about 1 minute. Gradually add remaining 1/4 cup sugar and whip until glossy, stiff peaks form, 2 to 3 minutes; transfer to third bowl.

Add flour mixture to now-empty mixer bowl and mix on low speed, gradually adding buttermilk mixture and mixing until almost incorporated (a few streaks of dry flour will remain), about 15 seconds. Scrape down bowl, then mix on medium-low speed until smooth and fully incorporated, 10 to 15 seconds.

Using rubber spatula, stir one-third of whites into batter. Gently fold remaining whites into batter until no white streaks remain. Transfer batter to prepared pan and smooth top with rubber spatula. Gently tap pan on counter to settle batter. Bake until toothpick inserted in center comes out clean, 28 minutes to 32 minutes, rotating pan halfway through baking.

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