Your sense of touch will tell you when the vanilla seeds are thoroughly mixed into the sugar. Also, you'll know the cake has finished baking when you press the surface and it springs back. Serve with fresh berries or fat-free whipped topping, if desired.
1 1/2 cups sugar, divided
1 (2-inch) piece vanilla bean, split lengthwise
1 cup sifted cake flour (about 4 ounces)
12 large egg whites
1/2 teaspoon cream of tartar
1/4 teaspoon salt
1 teaspoon lemon juice
How to Make It
Preheat oven to 325°.
Place 3/4 cup sugar in a small bowl. Scrape seeds from vanilla bean, and add seeds to sugar; discard bean. Work the vanilla seeds into sugar with fingers until well combined.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and sugar mixture, stirring with a whisk to combine.
Beat egg whites with a mixer at high speed until foamy. Add cream of tartar and salt; beat until soft peaks form. Add remaining 3/4 cup sugar, 2 tablespoons at a time, beating until stiff peaks form. Beat in juice. Sift flour mixture over egg white mixture, 1/4 cup at a time; fold in after each addition.
Spoon the batter into an ungreased 10-inch tube pan, spreading evenly. Break air pockets by cutting through batter with a knife. Bake at 325° for 50 minutes or until cake springs back when lightly touched. Invert pan; cool completely. Loosen cake from sides of pan using a narrow metal spatula. Invert cake onto plate.