Preheat oven to 350°F. Prepare two rimmed baking sheets with parchment paper. Spray parchment paper lightly with nonstick spray.
In a large bowl, whisk the powdered sugar, cocoa powder, salt, and baking powder. 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.
Spoon the batter onto the prepared baking sheets in 10 evenly spaced mounds per cookie sheet. I like to use a 1 ½ tablespoon spring-release cookie scoop.
Bake about 14 minutes, until the tops are glossy and lightly cracked. Slide the parchment paper (with the cookies) onto wire racks. Let cookies cool completely, and store in an airtight container for up to 3 days.
Notes
You may need only 2 egg whites, depending on the egg size. An average egg contains about 2 tablespoons of egg white.
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.