Move oven rack to lowest position and preheat oven to 375° F.

In a small bowl, whisk together powdered sugar and flour; set aside.

In the bowl of a stand mixer, beat egg whites and cream of tartar on medium speed until foamy. Make sure your bowl and beaters are very clean and dry and that there are no egg yolks present, as even a bit of grease or egg yolk will prevent egg whites from beating properly. Beat in granulated sugar on high speed, 2 tablespoons at a time, adding vanilla, almond extract, and salt with the last addition of sugar. Continue beating until soft peaks form that are slightly stiffened and glossy. Now you have meringue!

Sprinkle powdered sugar-flour mixture, ¼ cup at a time, over meringue, gently folding it in just until sugar-flour mixture disappears. Over-folding can break down the egg whites and result in a compact cake. Transfer batter to an ungreased 10" x 4" tube pan. Cut gently through batter with a butter knife to even out the batter.

Bake 30 to 35 minutes, or until cracks in cake feel dry and the top springs back when touched lightly. Immediately turn pan upside down onto a bottle neck or metal funnel on the countertop and let hang about 2 hours or until cake is completely cool. (A cake that pulls away and/or falls out of the pan is underbaked.)

Turn cake pan right-side-up and gently slide a thin knife around the cake's edge, then remove cake from pan. Serve with whipped cream, plus raspberry sauce, rhubarb sauce, or fresh berries.