Preheat the oven to 350°F, making sure the middle rack is in the lower third of the oven.
In a small bowl, sift and whisk together the flour, ¾ cup caster sugar, and cornstarch. Set aside.
Add the egg whites, salt, and cream of tartar to a large grease-free bowl. Use a stand mixer or hand mixer fitted with the whisk attachment to beat the egg whites until foamy. Add the vanilla and almond extracts.
Continue to beat the egg whites at medium speed until the egg whites have thickened and increased in volume. Add the remaining ¾ cup caster sugar, a few tablespoons at a time, until the meringue is shiny/glossy and holds a soft peak.
Set the mixer aside and use a silicone spatula to fold ¼ cup of the flour mixture into the meringue. Continue to fold ¼ cup of the flour into the meringue until all the flour mixture is incorporated.
Transfer the batter to an ungreased 10-inch tube pan (angel food cake pan) and spread it evenly in the pan. Tap the pan on the counter (gently) a few times to release any large air pockets. Bake in the lower third of the oven for 45 minutes or until the cake is golden brown and the top springs back when gently pressed. Using a cake tester is not recommended for angel food cake.
When done, remove the cake from the pan and invert the pan onto a metal rack if your angel food cake pan has feet. If not, invert the pan onto a glass bottle or prop up on small heat proof containers, such as ramekins.
Let the cake cool, upside-down, in the pan for at least 2 hours or until completely cool. Once cool, use a thin spatula (I use an offset spatula,) to loosen the cake from the pan and turn the cake onto a cake plate.
Use a serrated knife to slice. Serve immediately with desired toppings or wrap tightly in plastic wrap until ready to serve.
• I used King Arthur Gluten-Free Measure for Measure Flour in this recipe.
• You can make your own superfine sugar by processing granulated sugar in the food processor or purchase it online.