In a medium bowl, quickly whisk together the flour, baking powder, and salt until well combined. Set this aside.
In a large bowl, using an electric mixer, cream the softened butter and granulated sugar together until the mixture is light and fluffy – this incorporates air for a tender cookie! Beat in the egg until just combined, then stir in the vanilla and peppermint extracts.
Gradually add the prepped dry ingredients to your wet mixture, mixing on low speed until a soft, cohesive dough forms. Stop mixing as soon as it comes together to keep the cookies tender.
Gently divide your fresh dough into two equal halves. Leave one half plain for your white swirl. To the second half, add a few drops of red gel food coloring and mix until you achieve a vibrant, uniform red.
Wrap each ball of dough separately in plastic wrap and chill them in the refrigerator for at least 30 minutes. This key step makes the dough firm and much easier to handle without colors bleeding.
Once chilled, lightly flour your work surface. Roll the plain dough into a neat 10x12 inch rectangle, aiming for an even ¼ inch thickness. Repeat this process with the red dough, ensuring it's the exact same size and thickness.
Carefully lift the red dough rectangle and place it directly on top of the plain dough rectangle, aligning the edges. Gently press the layers together with your hands or a rolling pin to make them stick.
Starting from one of the long sides, carefully and tightly roll the layered dough into a log. As you roll, gently press to remove any air pockets, ensuring a consistently tight log for perfectly defined swirls.
Wrap the formed dough log tightly in plastic wrap and return it to the refrigerator for at least 2 hours, or until it's very firm. This longer chill is vital for slicing clean, crisp swirls that hold their shape.
When ready, preheat your oven to 375°F (190°C). Line your baking sheets with parchment paper to prevent sticking and promote even baking.
Remove the firm dough log from the fridge. For extra sparkle and crunch, lightly brush the log with water or a whisked egg white, then roll it evenly in coarse sugar or crushed peppermint candies.
Using a very sharp knife, slice the chilled dough log into uniform ¼-inch thick rounds. Place the sliced cookies about 1 inch apart on your prepared baking sheets.
Bake for 8 to 10 minutes, or until the edges are lightly golden. It's important not to overbake; these cookies will continue to firm up as they cool, keeping their tender texture.
Let the freshly baked cookies cool on the baking sheets for a few minutes before carefully transferring them to a wire rack to cool completely. Then, share and enjoy your beautiful, bakery-perfect peppermint swirls!