Step 1In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, brown sugar, vanilla and salt.
Step 2In a large bowl, sift together the flour, cocoa powder, cinnamon, nutmeg and baking soda. Add to the creamed mixture and mix until well combined. Mix in the chocolate and lemon zest. Add the egg white and mix just until blended.
Step 3Roll the dough into two 1 1/4 -inch diameter round logs and wrap in plastic film. Chill several hours or overnight. Cut the logs into rounds about three-eighths-inch thick.
Step 4Heat the oven to 350 degrees. In a small bowl, lightly whisk the whole egg. Place the sugar crystals in another small dish. Dip the sides only of each cookie into the egg and then in the sugar crystals. Place the sables on a parchment-lined baking sheet. Bake for about 10 minutes until slightly puffed in the center. Place the baking sheet on a rack and let cookies cool for 5 minutes before removing them from parchment paper.