Preheat the oven to 325 degrees.
Separate out the egg whites of 6 eggs (do this while they are cold) or measure out ½ cup (6 ounces) of carton egg whites.
Set the egg whites aside to allow them to warm up for 20-30 minutes.
While the egg whites warm, whisk together the dry ingredients in a medium bowl (cake flour, ½ cup granulated sugar, salt, cream of tartar).
Once the egg whites have sat out for 20-30 minutes whisk them on high in a stand mixer fitted with the whisk attachment or use a hand mixer (and a large bowl). Once the eggs become frothy, add ¼ cup granulated sugar. Beat on high until soft peaks form (about 4 minutes).
Sift the dry ingredients into the whisked egg whites (or slowly pour in small portions if you don't have a sifter). After each ¼-½ cup is added, gently fold the mixture together to incorporate the dry ingredients into the egg whites. Add the flour mixture slowly and in small portions, folding the mixture gently in between to avoid losing as much egg white volume as possible.
Gently spoon the mixture into an ungreased 9x5-inch loaf pan. Carefully spread the mixture evenly throughout the pan. Bake for 25-35 minutes or until the top is light brown and a toothpick comes out clean. Spin the cake after 15 minutes for even baking.
Once removed from the oven, immediately flip the pan over and allow the cake to cool upside down on a cooling rack (while still in the pan). Cool the cake to room temperature.
Once the cake has cooled, flip the pan back over and run a thin knife along the edges of the pan to release the cake.
Cut the cool angel food cake into 1-2 centimeter cubes and set aside.