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Place racks in lower and upper thirds of oven and preheat to 350 degrees.
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Whisk powdered sugar, cocoa powder, and salt in a large bowl, then whisk in egg whites and egg; fold in chocolate and cacao nibs. You should have a mostly smooth, thick batter.
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Spoon batter onto 2 parchment-lined or greased baking sheets, leaving enough space between them.
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Bake the cookies, rotating sheets from front to back once, 14–16 minutes.
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The finished cookies will be puffed, cracked, and set just around the edges.
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Let cookies cool on pan (they’ll firm up).
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Enjoy! :D