Add butter and sugar to the bowl of a stand mixer and beat on medium speed for 3–4 minutes, until creamy.

Add the egg and vanilla extract and beat well, scraping the bowl as needed.

In a separate bowl, whisk together the flour, salt, and baking soda.

With the mixer on low, gradually add the dry ingredients and mix until a soft dough forms.

Divide the dough into three equal portions.

Tint one portion with red gel food coloring, tint another with green, and leave the last portion plain.

Shape each portion into a ball, wrap in plastic wrap, and chill for 1 hour.

Roll each dough ball between two sheets of parchment into a 7-inch square. Chill for 15 minutes.

Stack the dough sheets: red on bottom, green in the middle, plain on top.

Roll into a 12×8-inch rectangle, then tightly roll into a log. Wrap and freeze for 1 hour.

Preheat oven to 350°F. Line baking sheets with parchment paper.

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

Place cookies 1 inch apart and bake for 8–10 minutes, until set.

Cool briefly on the baking sheet, then transfer to a wire rack to cool completely.

Store in an airtight container.