Preheat the oven to 350˚F. In a large bowl, whisk together the powdered sugar, gluten-free all purpose flour blend, xanthan gum (if using), cornstarch, and sea salt. Set aside.
In the bowl of a stand mixer fitted with the whisk attachment, add the egg whites. Beat on medium speed until the egg whites begin to look frothy. Add the cream of tartar and continue beating until the eggs hold soft peaks.
Then, while still beating at medium speed, gradually add the granulated sugar to the egg whites, about 1-2 tablespoons at a time. Mix thoroughly in between each addition. The total process will take awhile to add all of the granulated sugar slowly.
Add the vanilla extract and increase the speed to medium-high and beat until the egg whites are glossy and hold stiff peaks.
Sprinkle the dry ingredients over the egg whites. Gently fold the dry ingredients into the egg whites, being careful to not deflate the egg whites.
Transfer the angel food cake batter to an ungreased 10-inch tube pan. Smooth out the top with a spatula.
Place in the preheated oven on the lowest position and bake at 350F for 30-35 minutes, until the cake is lightly browned and springs back when you press a finger to it. Remove the pan from the oven and immediately invert the pan on a cooling rack so it is elevated. Let cool completely.
Once cool, run a knife along the edges (including the center ring) to loosen the cake from the pan. Invert the cake onto a serving platter and top with fresh whipped cream and berries before serving.