Preheat oven to 350 degrees.
In large bowl, add powdered sugar, cocoa powder, and salt. Stir.
Add egg whites and vanilla and stir well to combine.
Fold in chocolate chips and walnuts.
Chill for at least 30 minutes.
Spoon on parchment or silpat lined baking sheets (to prevent sticking).
Bake for 11-15 minutes or until they edges start to crack.
Let cool on baking sheets.
Sprinkle with sea salt, if so desired.
