Step 1Rinse the zucchini, grate and place in a colander. Drain for 15 minutes, then squeeze to remove any excess liquid; you should have 2 packed cups.
Step 2In a large bowl, mix together the egg yolks, feta cheese, green onions, flour, baking powder, basil and pepper. Add the drained zucchini, and mix well. Whip the egg whites till soft peaks form and fold into the feta mixture.
Step 3Heat an 8-inch heavy or nonstick frying pan over medium-low heat and add just enough oil to cover the bottom of the pan. Pour enough batter to cover the base of the pan, a scant one-half cup for each medium frittata or about 1 heaping tablespoon for each mini frittata. Cook the frittatas until they are golden brown on one side and set in the center, about 3 minutes (the mini frittatas will take less time), adjusting the heat as needed so they don't brown before they are set. Gently flip the frittatas over and lightly brown the other side.
Step 4Keep the finished frittatas in a warm oven until all the batter is used.