Step 1: Preheat your oven to 350°F (175°C). Do not grease a 10-inch two-piece tube pan. The batter needs to cling to the sides of the pan to rise.
Step 2: In a medium bowl, sift together 1/2 cup of the granulated sugar with the cake flour at least 2-3 times. Set aside.
Step 3: In a large, very clean mixing bowl (preferably metal or glass) using an electric mixer with a whisk attachment, beat the room temperature egg whites until foamy. Add the cream of tartar and salt, and continue beating until soft peaks form.
Step 4: With the mixer still running on medium-high speed, gradually add the remaining 1 cup of granulated sugar, about 1 tablespoon at a time, allowing it to incorporate fully after each addition. Continue beating until stiff, glossy peaks form. The meringue should be shiny and hold its shape well when the whisk is lifted. Beat in the vanilla extract.
Step 5: Sift about one-quarter of the flour-sugar mixture over the egg whites. Gently fold it in with a rubber spatula using an upward motion, turning the bowl as you go. Repeat with the remaining flour-sugar mixture, sifting in small batches and folding gently until just combined. Do not overmix.
Step 6: Gently spoon the batter evenly into the ungreased tube pan. Run a knife or thin spatula through the batter to remove any large air pockets.
Step 7: Bake for 35-40 minutes, or until the top is golden brown and springs back when lightly touched. Immediately invert the pan over the neck of a bottle or onto its tiny feet (if your pan has them). Let it cool completely upside down for at least 1.5 to 2 hours.
Step 8: Once completely cool, run a thin knife or offset spatula around the edges of the pan and the inner core to release the cake. Invert onto a serving plate. Slice with a serrated knife using a gentle sawing motion, or use two forks to gently pull apart slices.