If you don't want to buy a a whole package of superfine sugar, you can grind regular granulated sugar in a food processor for a few minutes to create your own superfine sugar.
If you don't have cake flour, you can substitute with all-purpose flour, but the cupcake will be a little more dense.
Finally, it is very important to GENTLY fold the flour mixture into the beaten egg whites. Never use an electric mixer to add the flour mixture at risk of deflating the egg whites.