Stir in two egg whites and vanilla extract; stir just until the batter is moistened. the cookie dough will look like brownie batter, thick and fudgy. If it seems too thick, stir in the other egg white. Fold in chocolate chips.
3 large egg whites, at room temperature, 2 teaspoons pure vanilla extract, 1 ½ cups semi-sweet chocolate chips
Spoon the batter onto the prepared baking sheets in 10 evenly spaced mounds per cookie sheet (makes 20 cookies total). I like to use a 1 ½ tablespoon spring-release cookie scoop. The cookies will spread quite a lot so allow room between them.
Bake about 14 minutes, until the tops are glossy and lightly cracked. Carefully slide the parchment paper with the cookies onto wire racks. Let cookies cool completely on the parchment.
Notes
Egg whites: You may only need 2 egg whites, depending on the egg size. An average egg contains about 2 tablespoons of egg white. It's easier to separate eggs when they're cold. The yolks are firmer and less likely to break. Once you separate the eggs, let the whites come to room temperature (it won't take long), and refrigerate the yolks for another use.
Variations: Add 1 tablespoon instant espresso powder, or 1 teaspoon cinnamon with the dry ingredients. Add a half cup chopped nuts. Lightly sprinkle coarse sea salt on tops of cookies before baking.
Storage: Store completely cooled cookies in an airtight container with parchment paper or waxed paper between each layer. They'll keep for about 3 days.