Step 1: In a large bowl, cream together the softened unsalted butter and granulated sugar using an electric mixer until light and fluffy (about 2-3 minutes).
Step 2: Beat in the egg and peppermint extract until well combined, scraping down the sides of the bowl as needed.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 5: Divide the dough in half. Leave one half plain and gently knead a few drops of red gel food coloring into the other half until evenly colored.
Step 6: Form each dough half into a disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes to chill.
Step 7: Preheat your oven to 375°F (190°C) and line baking sheets with parchment paper.
Step 8: On a lightly floured surface, roll out one disc of plain dough into a rectangle, approximately 1/8 to 1/4 inch thick.
Step 9: Repeat with the red dough, rolling it out to the same size and thickness.
Step 10: Carefully place the red dough rectangle on top of the plain white dough rectangle, aligning the edges and gently pressing to remove air pockets.
Step 11: Starting from one of the longer sides, tightly roll the layered dough into a log.
Step 12: Wrap the log tightly in plastic wrap and refrigerate for another 30 minutes to 1 hour to firm up.
Step 13: Once chilled, unwrap the dough log and use a sharp knife to slice it into 1/4-inch thick rounds. Wipe the knife clean between slices for best results.
Step 14: Place the cookie slices about 2 inches apart on the prepared baking sheets.
Step 15: Bake for 8-10 minutes, or until the edges are lightly golden and the centers still look soft. Do not overbake.
Step 16: Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.