Wash the mixing bowl and beaters really well (if even a little fat is left, this can cause the egg whites not to whip). Put the egg whites and cream of tartar into the bowl and beat with the mixer on high until frothy. Gradually add the remaining sugar and continue beating until stiff peaks form (the whites will stand up and look glossy, not dry). Fold about one-third of the whites into the batter, then the remaining whites. Don't worry if you still see a few white specks, as they'll disappear during baking. Gently spread out the batter over the bottom of the pan, and bake just until set and golden (not wet or sticky), about 10-12 minutes. Touch the cake gently in the center. If it springs back, it's done. Watch carefully and don't let the top brown. Leave the crust in the pan and place on a wire rack to cool. Leave the oven on while you prepare the batter.