Preheat the oven to 375°F. Line one large baking sheet with parchment paper.
Dice the lemon or citron peel. Set aside.
In a medium mixing bowl, combine the powdered sugar, cocoa powder, and salt. Stir until the ingredients are well blended together.
Zest the limes. Combine the zest with the egg whites in a separate bowl. Whisk for 30 seconds.
Add the egg mixture to the cocoa mixture. Stir until the dry ingredients are completely moistened. The batter will be dark and very gooey.
Spoon approximately 2 tsp worth of batter onto the prepared baking sheet for each cookie. Leave 2 inches between cookies. Place a piece of lemon or citron peel in the center of each cookie.
Bake for 12 minutes. The cookies will be glossy on the top.
Transfer the parchment paper, with the cookies still on it, to a cooling rack. After 3 minutes the cookies should easily peel off of the parchment paper. Cool the cookies to room temperature on a cooling rack.