01 - Whisk together the flour, baking powder, and salt in a medium bowl. Set aside for later use.
02 - Beat the softened butter and granulated sugar with an electric mixer until light and fluffy, approximately 2-3 minutes.
03 - Mix in the egg, vanilla extract, and peppermint extract until fully incorporated into the butter mixture.
04 - Gradually add the dry ingredients to the wet mixture, mixing just until combined. Be careful not to overmix.
05 - Split the dough evenly into two portions. Tint one portion with red gel food coloring, kneading gently until the color is uniform throughout.
06 - Flatten both dough portions into rectangles, wrap in plastic wrap, and refrigerate for at least 30 minutes.
07 - Roll out each chilled dough portion separately on parchment paper to about 1/4-inch thickness.
08 - Place one dough layer on top of the other and press gently together. Trim edges to form a neat rectangle if necessary. Starting from the long side, tightly roll the dough into a log to create the swirl pattern. Wrap and refrigerate for 20 minutes to firm up.
09 - Preheat oven to 350°F. Line baking sheets with parchment paper.
10 - Remove the dough log from plastic and roll in crushed peppermint candies, pressing gently to adhere the coating.
11 - Cut the log into 1/4-inch rounds and place 2 inches apart on the prepared baking sheets. Sprinkle with sparkling sugar if desired.
12 - Bake for 10-12 minutes, until the edges are just set and very lightly golden.
13 - Allow cookies to cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely before serving.