Wash mushrooms gently, remove the stem and cut lengthwise. Dip mushrooms in the egg whites, drain.In a small bowl, mix salt, flour, corn flour and baking powder. Put the mushrooms in the flour mixture, make sure all are fully coated. Heat oil in frying pan and deep fry mushrooms until crisp.Remove from heat and drain over kitchen towel, let cool. Sprinkle with garlic salt and toss gently.Keep in airtight container.