Heat the oven to 350 degrees. Line two baking sheets with parchment paper.
Melt the butter in a small pot over medium heat, swirling, until it's starting to foam and brown, 3 to 4 minutes. Let cool.
Whisk the confectioners’ sugar, cocoa powder and salt in a medium bowl, ridding it of as many lumps as possible.
Using a spatula, mix in the egg whites, whole egg and browned butter, stirring until you’ve got a good, smoothish mixture, followed by the chocolate and any nuts you may want to add.
Using a spoon, drop quarter-size blobs of dough onto the baking sheet, spacing them about 2 inches apart.
Sprinkle with flaky salt and bake until the cookies have flattened considerably and look baked through and a little wrinkled, 6 to 8 minutes.
Let cool before eating so they can firm up.
