In a bowl, whisk together 1 3/4 cups all-purpose flour, 1/2 teaspoon kosher salt, and 1/4 teaspoon baking soda. Set aside.

In a large mixing bowl, beat 2/3 cup softened unsalted butter until creamy (1-2 min). Gradually add 2/3 cup sugar, beating until light and fluffy (3-5 min), scraping bowl sides.

Beat in 1 egg until smooth, then stir in 1 teaspoon vanilla extract. Beat for another minute until thoroughly mixed.

Gradually add the flour mixture to the wet ingredients on low speed, mixing just until no dry streaks remain. Do not overmix; the dough will be soft and pliable.

Divide the dough into two equal halves. Mix red gel food coloring into one half until uniformly red. Mix green gel food coloring into the other half until uniformly green.

Shape each colored dough into a 1-inch thick disc. Individually wrap each disc tightly in plastic wrap and refrigerate for at least 2 hours (or up to 4 hours).

Remove one disc from the fridge and let sit for 10-15 min to soften slightly. Lightly flour a surface or use parchment paper. Roll the dough into a 12x9-inch rectangle, about ⅛-inch thick. Repeat with the other dough disc.

Carefully transfer one rolled dough rectangle onto a fresh piece of parchment paper. Gently lay the other dough rectangle directly on top, aligning the edges perfectly. Press lightly to adhere them without air pockets.

Starting from one of the longer 12-inch sides, tightly roll the layered dough into a snug log. Press along the seam to seal securely. If desired, sprinkle red sanding sugar on top of the log, pressing gently to adhere.

Wrap the dough log tightly in plastic wrap. Refrigerate for at least 1 hour (or up to 2 hours) to firm up.

About 15-20 minutes before baking, preheat the oven to 375°F (190°C). Line two large baking sheets with parchment paper or silicone mats.

Remove the chilled dough log from the refrigerator. Using a sharp, non-serrated knife, slice the log into rounds about ¼-inch thick. Place slices about 1 inch apart on the prepared baking sheets.

Bake for 8-10 minutes, or until the edges are lightly golden. Cookies will seem soft in the center but will firm up as they cool.

Let the cookies cool on the baking sheets for about 5 minutes before carefully transferring them to a wire rack to cool completely. Store in an airtight container at room temperature for up to 5-7 days.