How to Make It
Preheat oven to 375°.
To prepare cake, weigh or lightly spoon flours and cornstarch into dry measuring cups; level with a knife. Combine flours, cornstarch, 3/4 cup sugar, and xanthan gum in a medium bowl; stir with a whisk.
Place egg whites in a large bowl; beat with a mixer at high speed until foamy. Add cream of tartar and salt; beat until soft peaks form. Gradually add 3/4 cup sugar, 2 tablespoons at a time, beating until stiff peaks form. Add vanilla; beat well. Sift flour mixture over egg white mixture, 1/4 cup at a time; fold in.
Spoon batter into an ungreased 10-inch tube pan. Break air pockets by cutting through batter with a knife. Bake at 375° for 45 minutes or until cake springs back when lightly touched and small cracks form on top of cake. Invert pan (but don’t remove from pan); cool completely. Loosen cake from sides of pan using a narrow metal spatula. Invert cake onto a plate. Cut cake into 24 slices.
To prepare compote, combine berries, 1/4 cup sugar, and 2 tablespoons water in a medium saucepan. Bring to a simmer over medium heat; cook 8 minutes or until berries soften and juices thicken slightly. Cool 10 minutes. Spoon compote and whipped topping over cake.
If your angel food cake pan doesn't have feet to set on for cooling, invert the pan on the neck of a wine bottle.