In a medium mixing bowl, cream together butter and sugar until pale and fluffy. Add egg and vanilla (or peppermint extract) and beat well. Sift the flour and baking soda together and mix the dry ingredients into the butter/sugar mixture.
Divide the dough into two sections. Then, wrap it in plastic kitchen wrap and chill until ready to use. Dough can be used right away but may be easier to work with if chilled first.
Preheat your oven to 350°. On a lightly floured surface roll out cookie dough to ¼” thickness. Use a square cookie cutter (I used a 2 ½” square cutter here) to cut out cookies. Transfer cookies to a baking sheet lined with parchment paper, leaving an inch of space around each cookie.
Bake cookies for 8 minutes or until the bottoms show just a bit of browning. Cool the cookies on baking sheet for a few minutes before transferring them to a cooling rack. Allow the cookies to cool completely before frosting.
Repeat rolling out, cutting, and baking cookies into 16 cookies.
For the icing
Use the whisk attachment of a standing mixer to beat together powdered sugar, meringue powder, 8 tablespoons of water, and glycerin until extremely fluffy - about 6-8 minutes.
Add additional water, 1 tablespoon at a time until icing is just thin enough to drizzle from the whisk and smooth out completely within 5-10 seconds. Spoon some of the icing into a shallow bowl.
Dip the top side of each cookie down into the icing, swishing back and forth gently to fully cover the top of the cookie and allow any excess to drip back into the bowl.
Flip the cookie right side up, use an offset spatula to scrape off any icing on the sides of the cookie and set cookie on a sheet of parchment paper to dry.
If the icing runs off of the cookie, it is too thin. Whisk in additional powdered sugar until the consistency is correct. If the icing does not smooth out after a few seconds, whisk in additional water until the consistency is correct.
Allow cookies to dry for 12-24 hours before painting. Mix approximately ⅛ teaspoon of red luster dust with 1 teaspoon of peppermint extract (you can also use vodka or another alcohol based clear extract). Place cookies in a 4x4 square, edges touching. Use paintbrushes of various widths to paint stripes across the cookie rows. Mix additional “paint” as needed. Stripes will dry almost instantly as alcohol evaporates. Rearrange cookies randomly to create an offset stripes pattern.
Leftover royal icing will stay fresh and useable for months if stored in an airtight container.
Thin smaller amounts of icing directly in the dipping bowl if you wish to preserve the thicker consistency for piping or making other icing decorations.
Once icing has dried, cookies should be stored in an airtight container for 3-4 days.
Cookie dough can be stored in the refrigerator or freezer before using. If dough is too soft for cookies to hold shape while cutting and transferring to baking sheet, chill dough for 20-30 minutes until it is more firm.
Luster dust colors can be combined to mix a custom color - I mixed a bright red with a burgundy here.
Glycerin is added to the royal icing to make it “glossy” icing, but I find there is not a big difference. Therefore, you can omit the glycerin from the recipe and still get beautiful results.
If you don't have meringue powder on hand, you can substitute it for two egg whites. Just reduce the amount of water that is added to the royal icing, in order to get the proper consistency.