In the bowl of an electric mixer, mix together the first 4 ingredients (through salt). Add egg whites and vanilla; beat on medium speed until homogenous and barely fluffy (about 3 min.). Add chocolate; mix by hand just until evenly distributed. Line 2 extra-large baking sheets with parchment paper; use a #40 cookie scoop to drop 10-11 evenly sized mounds of batter (about 2 oz. or 2 tbsp. each) 3" apart on baking sheets. Bake in a preheated 350° oven until just barely set and puffy (11-12 min.). Remove from oven; lift parchment and cookies to a wire rack (do not remove parchment from cookies until they are completely cool). Gently peel cookies off parchment; store in an airtight container at room temperature for up to 1 week.