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Lightly grease two baking sheets. Or line with parchment, and grease the parchment. Yes, grease the parchment; these cookies are sticky, and need to be baked on a greased surface.
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Whisk together the egg whites and vanilla. In a separate bowl, whisk together the dry ingredients, except for the chips/nuts/fruit.
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Stir the wet and dry ingredients together. Scrape the bottom and sides of the bowl, and stir again until smooth. The batter should be and the consistency of a thick syrup. If it's not, add more egg white until it drips from a spatula in a thick ribbon.
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Add the chips and/or nuts, if you're using them.
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Drop the syrupy batter onto the prepared baking sheets in 3" circles (for large cookies), or 1 3/4" to 2" circles (for smaller cookies); a tablespoon cookie scoop or teaspoon cookie scoop, respectively, work well here. Let the cookies rest on the baking sheets for 30 minutes, while you preheat your oven to 350°F.
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Bake the cookies for 7 minutes (for smaller cookies), 8 to 9 minutes for the larger cookies; they should spread slightly, become somewhat shiny, and develop faintly crackly tops. Note: large cookies with added chips/nuts will need to bake for 10 minutes.
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Remove the cookies from the oven, and allow them to cool right on the pan. When they're nearly cool, carefully loosen them from the pan with a spatula.
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Storage information: Serve warm; or cool completely, and store airtight at room temperature for several days. For longer storage, wrap well and freeze.