Preheat the oven to 350°F.
Whisk together the powdered sugar, salt, cocoa, and cornstarch in a large bowl. Add egg whites and vanilla and stir until a thick mixture forms. It will look like it’s not enough liquid - just keep stirring, but do it carefully so you don’t spill cocoa mixture all over the place. The batter will eventually come together. It’s very, very thick. Stir in the baking chunks and nuts, if using.
Line two baking sheets with parchment paper and spray with nonstick cooking spray. Yes, even though you’re using parchment you need to spray it or grease it. You can use silicone baking mats, but spray them too. (I had better luck with parchment, but the silicone mats work just fine.)
Scoop 2 tablespoon sized balls of dough onto the cookie sheets, leaving a few inches between them. Place M&Ms all over the top of the cookies. The cookies will spread, so add as many as you can fit (about 5-7).
Bake for 11-13 minutes until most of the cookie is no longer glossy. There may still be a spot or two of glossy in the center. Let cool completely before removing from the pan using a spatula. Store in an airtight container for up to 3 days.