Step 1 In a medium bowl, beat the cornstarch gradually into the egg whites, then whisk in the water bit by bit until you have about 1 1/2 cups of batter.
Step 2 Heat a 9-inch skillet over low heat until hot. Rub a little hard-boiled egg yolk all over the pan using a folded paper towel (the egg will be crumbly, so brush off any large bits). This will help keep the wafers from sticking to the pan. Add about one-fourth cup of the batter into the pan, swirling around until the base is covered. Cook until the sides start to curl inward and the wafer releases from the pan, 1 to 2 minutes, depending on the heat of the pan (it will look much like a crepe). The wafers should not color but remain white. Remove the wafer to a plate or baking sheet.
Step 3Repeat with the remaining batter to make 6 wafers. Set aside.