Adjust the oven rack to the lower-middle position and preheat the oven to 350°.
In a large bowl using a hand mixer or a stand mixer fitted with a whisk attachment, whip egg whites and cream of tartar together on medium speed until foamy, about 1 minute.
2 - 2 1/4 c. egg whites (about 12 egg whites), room temperature, 1 1/2 tsp. cream of tartar
Switch to medium-high and slowly add the granulated sugar. Whip until soft peaks form. Add the vanilla extract, then beat just until incorporated.
1 c. white sugar, 1 tsp. vanilla
Using a fine-mesh strainer, sift together the powdered sugar, cake flour, and salt.
1 1/2 c. confectioner's sugar, 1 c. cake flour (* see notes for substitution), 1 tsp. salt
In 3 additions, slowly sift the flour mixture into the egg white mixture, gently folding with a rubber spatula after each addition. To avoid deflating or a dense cake, don’t add the flour mixture all at once. Sift and very slowly fold in several additions. Handling with care, pour and spread the batter into an ungreased 10" tube pan.
Bake the cake for about 40 - 45 minutes, until the top looks dry and springs back when lightly touched.
Remove from the oven and invert the cake pan onto its stilts or a wire rack to cool. (Upside-down so the bottom of the tube pan is right-side up.)
Once cooled, run a thin knife around the outside and inside edges to loosen the cake. Remove the outside of the pan (if you have a 2-piece pan). Run the knife along the bottom and transfer it onto a large plate.
If desired, dust with confectioners’ sugar. Slice the cake in sawing motion with a sharp serrated knife. Regular knives can easily squish the cake.
Serve with whipped cream and fresh berries. Or cut the cake into cubes and use it to make trifles.