In a pie plate, beat the egg whites, egg, milk, salt and vanilla with a whisk until blended. In a 12-inch nonstick skillet, melt 1 teaspoon butter or trans-fat-free margarine on medium heat.
Dip the bread slices, one at a time, in the egg mixture, pressing the bread lightly to coat both sides well. Place 3 or 4 slices in the skillet, and cook until lightly browned, 3 to 4 minutes. Flip and cook until lightly browned on the second side, 3 to 4 minutes.
Transfer the French toast to a cookie sheet; keep warm in the oven. Repeat with remaining butter or margarine, bread slices and egg mixture.