gel food coloringI used red, electric pink, and, for some cookies, just a touch of royal purple
Preheat oven to 350F.
In KitchenAid mixer or with electric hand-mixer beat your butter until well-creamed.
Add sugar to the mixing bowl and beat with the butter until well combined (about 30 seconds).
Stir in vanilla extract and salt, stirring until well-combined, and scrape down the sides of the bowl using a rubber spatula to ensure ingredients are well-incorporated.
Add egg yolk and beat until combined
With mixer on medium-low speed, gradually add flour until well-incorporated (you will definitely need to scrape down the sides and the bottom of the bowl several times for this, as the mixture is fairly dry and crumbs tend to settle at the bottom of the bowl).
Transfer dough to clean surface and use your hands to form it into a cohesive ball.
Flatten into a disk (about 1" thick), cover with clear wrap, and chill for 15 minutes.
Once dough has chilled, transfer it to a clean, lightly floured surface and use a lightly floured rolling pin to roll out to about ¼" thickness.
Lightly flour your cookie cutters and cut out your shapes, transferring them to ungreased cookie sheet.
Once you have cut out all of the shapes that you can, be sure to re-combine the scraps and roll out and cut out again to get the maximum number of cookies from your dough.
Bake on 350F for 10-12 minutes (longer if you are using larger cookie cutterand allow cookies to cool completely before dipping in chocolate.
Marble Icing Glaze
In a large bowl, combine sifted powdered sugar, 3 Tbsp milk, and light corn syrup. Whisk until smooth. If mixture is too dry, add additional milk, one teaspoon at a time until mixture can be stirred.
Stir in extracts.
Glaze should be thick, to test that it is the right consistency drop a few droplets on top of a test cookie. If the glaze remains stiff, it needs to be thinned. If it runs off the cookie, it is too thin and needs more powdered sugar. If it settles down smoothly, it is just right.
Divide glaze into four small bowls.
Working with one bowl at a time, use a toothpick (a different toothpick for each color) and dip it into the food coloring bottle and then into the glaze. You want a small pool of food coloring about the size of a mini chocolate chip or two.
Drag a toothpick through the food coloring and glaze until it is prettily swirled/marbled.
Holding your cookie at the base, dip the surface of each cookie into the glaze (take a look at your first cookie -- are you satisfied with the pattern? Adjust whorls with additional food coloring as needed).
Once the first bowl runs out or the colors become too muddled together from dipping, move onto your next bowl of food coloring (you will probably need to stir as the surface may have begun to harden).
Repeat steps 5-7 until all cookies have been dipped. Allow them to harden at least 30 minutes (may take longer) for glaze to set. Enjoy!
Keep uneaten cookies in an airtight container at room temperature.