1. Preheat oven to 375°.
2. To prepare cake, weigh or lightly spoon white rice flour, cornstarch, sweet white sorghum flour, and almond meal flour into dry measuring cups; level with a knife. Combine white rice flour, cornstarch, sweet white sorghum flour, almond meal flour, 3/4 cup granulated sugar, cocoa, and xanthan gum in a medium bowl; stir with a whisk.
3. 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 granulated sugar, 2 tablespoons at a time, beating until stiff peaks form. Add almond extract; beat well. Sift flour mixture over egg white mixture, 1/4 cup at a time; fold in.
4. 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.
5. To prepare glaze, combine powdered sugar and remaining ingredients in a small bowl; stir with a whisk until smooth. Spoon over cake. Let stand 10 minutes or until glaze is set. Cut cake into 12 slices.