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
5 cups Vanilla Frosting:
- 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
- 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.
Let cake cool completely in pan on wire rack, about 2 hours.
While cake cools, assemble the frosting. Using stand mixer fitted with paddle, beat butter, cream, vanilla, and salt on medium-high speed until smooth, about 1 minute. Reduce speed to medium-low, slowly add sugar, and beat until incorporated and smooth, about 4 minutes.
Increase speed to medium-high and beat until frosting is light and fluffy, about 5 minutes. (Frosting can be refrigerated for up to 3 days; let soften at room temperature, about 2 hours, then re-whip on medium speed until smooth, 2 to 5 minutes.)
To assemble, line edges of cake platter with 4 strips of parchment to keep platter clean. Place cake on platter. Spread frosting evenly over top and sides of cake. Using blueberries, outline 6 by 4 1/2-inch rectangle in top left corner of cake. Make diagonal rows of blueberries within outline, leaving single blueberry's width between rows. Lay additional blueberries evenly between rows to make blueberry checkerboard. Gently press blueberries to adhere.
For red stripes, lay raspberries on their side and gently nestle them next to one another. There should be 4 short rows of raspberry stripes across top of cake and 3 long rows of raspberry stripes across bottom of cake. Gently press raspberries to adhere. Serve.
Nutrition information per serving: 765 calories; 379 calories from fat; 43 g fat (24 g saturated; 2 g trans fats); 199 mg cholesterol; 295 mg sodium; 92 g carbohydrate; 3 g fiber; 65 g sugar; 6 g protein.