• Preheat your oven to 350°F. Set a 10-inch nonstick tube pan (preferably a two-piece tube pan that has a removable bottom) to the side. Don’t grease or otherwise prepare it.

Sift the dry ingredients.

• You will sift the flour and cornstarch 2 times, then add the cornstarch and xanthan gum to the mixture and sift twice more. This is an essential step.
• Set a medium-size bowl and 2 pieces of parchment paper on a flat surface, along with a sifter.
• Place the flour blend and cornstarch in the sifter, and sift onto the parchment paper. Shake the flour blend and cornstarch off the paper back into the sifter and sift onto the other piece of parchment.
• Shake the flour blend and cornstarch off the paper along with the xanthan gum and ¾ cup (86 grams) of the confectioner’s sugar back into the sifter. Sift it all onto the parchment paper.
• Shake everything from the parchment paper back into the sifter and sift one final time (for a total of 4 times) into the mixing bowl.
• Add the salt, and whisk to combine.

Make the cake batter.

• In the bowl of a stand mixer fitted with the whisk attachment or a large bowl with a handheld mixer, beat the egg whites, warm water, cream of tartar and almond (or vanilla) extract until begging to foam (about 30 seconds).
• With the mixer on medium-high speed, add the remaining 3/4 cup (86 g) confectioner’s sugar in 3 or 4 batches, until soft peaks form (3 to 4 minutes).
• Once all the sugar has been added, increase the speed to high and beat until peaks become stiff and glossy, about another 2 minutes. The beater will begin to leave a trail in the whites, which are now a meringue. Remove the mixing bowl from the mixer.
• In 4 batches, add the sifted flour mixture to the meringue, gently folding in the flour by hand with a silicone spatula after each addition. Work quickly but carefully, so as not to deflate the egg whites. The mixture should be fluffy but relatively stable.
• Carefully transfer the cake batter to the ungreased tube pan. Run a butter knife or small offset spatula carefully through the batter in a careful circular motion to release any trapped pockets of air. Smooth the top with a silicone spatula. The pan will be nearly full.

Bake the cake.

• Place the pan in the center of the preheated oven, and bake until a cake tester inserted near the center comes out clean, the top is lightly browned, and it springs back when pushed gently, about 35 minutes. Don’t overbake.

Cool the cake.

• Invert the pan over a long-neck bottle if your pan doesn’t have legs to elevate it from the counter when inverted. Allow the cake to cool in the pan for at least 1 hour.
• Re-invert the cake so it’s right-side up. Coax the cake away from the sides and neck of the pan with a butter knife or offset spatula (ideally, plastic, so you don’t scratch the pan).
• If your pan is in two parts, with removable sides, press upward on the cake bottom and remove it from the rest of the pan. Run a straight edge along the bottom of the cake to separate it from the bottom of the pan.
• Place a wire rack on top of the cake, then invert both onto a wire rack. Remove the bottom of the cake pan. Allow the cake to finish cooling to cool to room temperature.
• Slice the cooled cake with a large serrated knife. For a clean cut, move the knife in one direction only, rather than sawing back and forth. Plate each slice with the optional berries and whipped cream and serve.
• Angel food cake freezes very well. You can wrap the whole, cooled cake tightly in freezer-safe wrap and freeze whole. You can also wrap individual slices tightly and freeze. Defrost at room temperature before serving.