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.
IF YOU WANT SLICE-AND-BAKE COOKIES...
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 distribute the dough log evenly. Twist up the ends of the parchment and refrigerate the dough overnight.
When you're ready to bake, 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.
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.
IF YOU WANT CUT OUT COOKIES...
Form the dough into a disc. Wrap in plastic wrap or beeswax wrap and refrigerate overnight.
Lightly dust your work surface with flour, and then roll your cookie dough out to about a ¼-inch thick. Using any shape cookie cutter, cut out cookies and place them on a cookie sheet with about an inch between each one. Reform the dough and roll it out again and again, lightly dusting with flour as needed. Cut out cookies until you can't anymore.
Bake your cookies one sheet at a time for 12 minutes for cookies approximately 3 inches wide. For smaller cookies, start at 8 minutes. For larger cookies, check at 12, but you may need to go to 15 minutes.
Allow the cookies to cool on the sheet for 5 minutes before transferring to a wire rack to cool completely before icing.
Mint Royal Icing
In a large mixing bowl, combine powdered sugar, meringue powder, water, and extracts. Whisk thoroughly.
Continue to whisk until icing slowly trails off whisk in a continuous ribbon. It should all run back into a smooth surface in about 10-15 seconds.
Transfer roughly ¾ of the icing to a piping bag. To the remaining icing, add in red food coloring for your peppermint swirl design. Transfer the red icing to a separate piping bag. (Tip: I use twist ties to close off my bags)
Snip a small corner off the white icing bag. Begin piping circles onto each cookie. Then, snip the corner a little larger and flood each cookie.
For the red peppermint swirl, snip a small tip off the red icing bag. Pipe an X and then an additional line through the X on each cookie (so there are 3 lines through the center). Then use a toothpick to swirl the icing starting in the center and working your way out.