Cream the unsalted butter and sugar in a stand mixer on medium speed for about 3-4 minutes until light and fluffy.

Add the egg and vanilla, mixing on low speed for about 30 seconds.

In a separate bowl, whisk together the flour, salt, and baking soda, then gradually add to the wet ingredients while mixing on low.

Divide the dough into thirds and color one third red, another green, and leave the last portion plain. Refrigerate for at least 1 hour.

Roll each piece of dough into 7-inch squares between parchment paper.

Stack the colored dough squares: red on the bottom, green in the middle, and plain on top. Roll into a log, wrap in plastic, and freeze for 1 hour.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Slice the chilled dough into 1/3-inch thick cookies and roll the edges in red sanding sugar.

Bake the cookies for 8-10 minutes until the edges are set but the centers are soft.

Cool on sheets for a few minutes before transferring to a wire rack to cool completely.