Combine cake flour and cornstarch in a bowl and set it aside.
Combine 10 grams of sugar with egg yolk in a small mixing bowl and beat with an electric mixer on high speed until thick and color becomes pastel yellow. Set it aside.
Add egg white in another mixing bowl, beat with a clean and dry electric mixer on medium speed until foamy. Add half of the sugar and continue to beat until the volume increases and air bubbles become smaller. Add the rest of the sugar and beat until the foam can form firm peaks (meaning that lifting the mixer out will result in egg white foam that stands straight up with the peak curling down just a bit).
Add about a half cup of the egg white into the egg yolk, whisk to combine, then pour the mixture back into the rest of the egg white and whisk slowly by hand until well combined.
Sift half of the flour mixture onto the surface of the egg mixture evenly, then fold the flour in with a spatula 5 times before sifting in the rest of the flour mixture.
Fold the rest of the flour in with the the spatula until just combined (no visible flour). The mixture will deflate a bit and look a bit rough, but do not try to mix until the mixture becomes smooth.
Transfer the mixture into a piping bag, cut a half inch wide tip and pipe out 1.5 inch diameter circles onto a lined baking sheet. You will get about 50 dollops.
Dip your finger with water and gently press down the tip of each dollop of batter.
Dust all the dollops with confectioner sugar for 2 times.
Bake for 11 - 13 minutes until the color starts to change, take it out, remove the baking mat or parchment paper from the pan and let it cool for few minutes before peeling the wafers off. Avoid over baking or your wafers will be dry and crunchy. A successful wafer should have a texture more similar to cake than to cookies.