Cream together the softened unsalted butter and sugar in a stand mixer until light and fluffy. Add egg and vanilla extract and mix until smooth.

In a separate bowl, whisk together flour, kosher salt, and baking soda. Gradually add to the creamed mixture and mix until just combined.

Divide the dough into three portions. Color two portions with red and green gel food coloring and leave one plain. Knead until colors are evenly distributed.

Shape each portion into balls, wrap in plastic wrap, and chill in the refrigerator for at least 1 hour.

Roll each colored dough between parchment paper into 7-inch squares and chill for an additional 15 minutes.

Stack the colored layers, rolling them into a tight log. Maintain uniform thickness throughout the roll.

Preheat the oven to 350°F (175°C). Slice the log into 1/3-inch thick rounds and roll edges in sanding sugar. Bake for 8-10 minutes.

Cool the cookies on the baking sheet for 5 minutes before transferring to a wire rack.