Preheat oven to 350℉ and line two baking sheets with parchment paper. Whisk powdered sugar, cocoa powder, and salt in a stand mixer. Add egg whites and egg. Combine, scraping the sides to incorporate. Fold in chocolate and cacao nibs. Use tablespoon to spoon batter onto prepared sheet pans, 2 inches apart. The cookies will spread out quite a bit while baking as the chocolate melts.
Bake cookies 7 minutes, then rotate the pans. Bake another 7 to 9 minutes until puffed, cracked, and just the edges are set. Cool on a wire rack where they will firm up after 15 minutes.
If you do not have cacao nibs or dutch process cocoa, please see this peanut butter brownie cookie instead.Do Ahead: Bake cookies 3 days ahead and store in an airtight container at room temperature.Note: Unlike other cookie dough, this will be a wet, sticky batter that will spread into nice round cookies during baking.