In a stand mixer, cream 18 tablespoons of softened salted butter with ¾ cup of powdered sugar until light and fluffy, about 3–5 minutes.

Gradually blend in 2¼ cups of all-purpose flour until just combined.

Divide the dough in half; mix ¾ teaspoon of peppermint extract into one half and 2 tablespoons of cocoa powder into the other.

Shape each dough into disks, wrap in plastic wrap and refrigerate for at least 30 minutes.

Lay each chilled dough between two sheets of parchment paper and roll them out to approximately 9x12 inches.

Tear the rolled chocolate dough into smaller pieces and scatter them over the rolled vanilla dough, gently folding to create marbling.

Wrap the marbled dough in plastic wrap and return it to the refrigerator for another 30 minutes.

Preheat oven to 350°F (175°C). Remove dough from fridge and slice it into ¼-inch rounds.

Place cookie slices on a parchment-lined baking sheet and bake for 16–20 minutes until edges are lightly golden.

Allow cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack.