In a stand mixer, combine 18 tablespoons of softened salted butter and ¾ cup of powdered sugar. Beat together until fluffy, about 2-3 minutes. Gradually blend in 2¼ cups of all-purpose flour until just combined. Divide the dough into two parts, mixing ¾ teaspoon of peppermint extract into one half and 2 tablespoons of cocoa powder into the other.

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

Roll out both the chocolate and vanilla doughs between sheets of parchment paper to about 9x12 inches.

Tear the rolled chocolate dough into pieces and scatter over the vanilla dough. Gently fold to create a marbled effect. Return to the fridge for another 30 minutes.

Preheat oven to 350°F (175°C). Slice chilled dough into ¼-inch rounds and place on a baking sheet, leaving space between each cookie. Bake for 16-20 minutes until edges are lightly golden.

Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.