Cream 1 cup of softened unsalted butter and 1 cup of sugar in a mixing bowl for 3-4 minutes until light and fluffy. Add 1 egg and 2 teaspoons of vanilla extract, mixing until smooth.

In another bowl, whisk together 2 ½ cups of all-purpose flour, ½ teaspoon of kosher salt, and 1 teaspoon of baking soda. Gradually incorporate this into the butter mixture until a soft dough forms.

Divide the dough into three portions. Add red gel food coloring to one, green to another, and leave the last portion plain. Wrap each colored dough and chill for 1 hour.

Roll each portion between parchment paper to ¼ inch thick. Stack the red and green on the plain dough, then roll into a log shape. Wrap tightly and freeze for 1 hour.

Preheat the oven to 350°F (175°C). Slice the log into 1/3-inch thick rounds to reveal the colorful swirls.

Roll the edges of each slice in red sanding sugar, place on a parchment-lined baking sheet, and bake for 8-10 minutes until set but not browned.

Let the cookies cool on the baking sheet for 1 minute before transferring to a wire rack to cool completely.