In a large mixing bowl, sift together sugar, cocoa and salt; stir in walnuts. Add egg whites and vanilla; stir until just moistened. Drop ½ of the batter by 1 tbsp. scoops 1 ½ - 2" apart on 2 parchment-lined baking sheets. Bake in a preheated 350° oven until tops are lightly cracked and glossy (about 15 min.), rotating and switching pans halfway through. Let rest on sheet pan 2 min; move cookies and parchment sheets to a wire rack to cool. When cool, gently peel paper from the backs of the cookies; repeat with remaining batter. Store in an airtight container at room temperature up to 1 week.