Preheat the oven to 350°F. Line two baking sheets with silpats or lightly greased parchment paper. Set aside.

Use a stand mixer fitted with a whisk attachment (or a large mixing bowl and a whisk) to quickly whisk together the confectioner's sugar, cocoa powder, and kosher salt until no more clumps remain. Beat in the egg whites and the eggs until there are no dry pockets and the mixture is glossy and smooth. It is easier with a stand mixer because it gets a bit gluey, but it can be done by hand, too!

Stir in the M&Ms and chocolate chunks or chocolate chips. Using a cookie scoop (disher) or a tablespoon, spoon the cookie batter onto the prepared pans, leaving at least 2-inches between each mound of batter as it will spread quite a bit.

Bake for 14-16 minutes, rotating the pans halfway through baking, or until the edges are set up and firm and the tops have a dull sheen and crackly surface. Remove the pans from the oven and let the cookies set up and cool completely on the pans. It's easiest to get them off of the silpats or parchment using a spatula.

Store in an airtight container at room temperature for up to 3 days.