In a stand mixer, cream together 18 tablespoons of softened salted butter and ¾ cup powdered sugar until light and fluffy for about 2-3 minutes. Gradually mix in 2 ¼ cups all-purpose flour until clumps form. Split this mixture in half, adding ¾ teaspoon peppermint extract to one half and 2 tablespoons cocoa powder to the other, blending until smooth and well combined.

Shape each dough into a disk, wrapping them separately in plastic wrap. Chill the doughs in the refrigerator for about 30 minutes, allowing them to firm up.

Once chilled, place each dough between two pieces of parchment paper and roll them out to approximately 9x12 inches for the vanilla dough and thinner for the chocolate dough.

Tear the chocolate dough into irregular pieces and scatter them over the vanilla dough. Gently fold, roll, and twist the layers together to achieve a marbled effect.

Preheat your oven to 350°F (175°C). Once the dough is firm, slice it into ¼-inch rounds.

After baking, let the chocolate peppermint marble cookies cool on the baking sheet for about 5 minutes. Transfer the cookies to a wire rack to cool completely.