Cream butter and sugar together in a stand mixer for 3–4 minutes until light and fluffy.

Add the egg and vanilla extract, and mix well, scraping down the bowl.

Whisk together flour, kosher salt, and baking soda in a separate bowl.

Gradually mix the dry ingredients into the butter mixture on low speed.

Divide the dough into three equal parts; color one red, one green, and leave the last part plain.

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

Roll each dough between sheets of parchment paper into 7-inch squares; chill again for 15 minutes.

Stack the colored dough layers: red, green, then plain, and roll into a 12×8-inch rectangle.

Remove the top parchment paper and tightly roll the dough lengthwise into a log.

Wrap the log and freeze for 1 hour.

Preheat the oven to 350°F and line baking sheets with parchment paper.

Slice the dough log into ⅓-inch cookies, and roll the edges in red sanding sugar.

Place the cookies 1 inch apart on baking sheets and bake for 8–10 minutes.

Cool the cookies on the baking sheet for 1 minute before transferring to a wire rack.

Store cooled cookies in an airtight container.