Sift together the flour, cocoa powder, salt, and baking soda.
Cream the room temperature butter with the sugar until fluffy. Then mix in each egg, one at a time, followed by the vanilla extract.
Incorporate the dry mixture, but do not over mix.
Dump the dough onto a piece of parchment paper, aiming to get it in a log shape. Roll it in the paper, and then roll the bundle like a snake to evenly distribute the dough log. Twist up the ends of the parchment and refrigerate the dough overnight.
Preheat the oven to 350ºF.
Unwrap your dough, and place on a floured surface. Gently roll the dough again to make sure the edges are smooth. Slice into 24 cookies. Refrigerate slices until they are ready to go in the oven.sd
Set 1 dozen cookies on a lined baking sheet and bake for 12 minutes. Move to a cooling rack, and repeat with the remaining dough. Allow them to cool completely.
Mint Royal Icing
In the mixer, combine the egg white, powdered sugar, extracts, and coloring.
Continue to whisk until stiff, glossy peaks form.
Transfer to a piping bag and snip off a small corner (roughly, an ⅛-inch hole). Ice your cookies wtih a zig-zag drizzle. Turn the cooling rack 90º and repeat the drizzle.