In a large mixing bowl, beat together unsalted butter and sugar until light and fluffy, about 3-4 minutes. Add egg and vanilla extract, mixing until incorporated.

In a separate bowl, whisk together all-purpose flour, kosher salt, and baking soda. Gradually incorporate into the creamed butter mixture until soft dough forms.

Divide dough into three parts. Add red and green food coloring to two portions, kneading until the colors are vibrant.

Wrap each portion in plastic wrap and refrigerate for about 1 hour.

Roll out each colored dough on parchment paper into squares approximately 7 inches wide. Stack the colors and roll into a tight log.

Wrap the log in parchment paper and freeze for about 1 hour.

Preheat oven to 350°F (175°C). Slice the log into 1/3-inch rounds, roll edges in red sanding sugar, and place on parchment-lined baking sheet. Bake for 8-10 minutes until edges are golden.

Let cookies cool on the baking sheet for 1 minute, then transfer to a wire rack to cool completely. Store in an airtight container.