Combine 12 tbsp salted butter, ½ cup powdered sugar, and 1 ½ tsp vanilla in a stand mixer. Beat until smooth, then add 1 ½ cups all-purpose flour until crumbly. Shape into a disk, wrap in plastic, and refrigerate for 30 minutes.

Clean the mixer bowl. Cream together 6 tbsp salted butter, ¼ cup powdered sugar, and ¾ tsp peppermint extract. Sift in cocoa powder and ¾ cup flour, mixing until dough forms. Shape, wrap, and chill for 30 minutes.

Roll out vanilla dough between parchment paper to 9x12 inches. Do the same with chocolate dough. Tear chocolate dough into irregular pieces and place on top of vanilla.

Fold chocolate pieces into vanilla dough. Roll gently into a log shape. Wrap and chill for at least 30 minutes.

Preheat oven to 350°F (175°C) and line baking sheets with parchment. Slice chilled dough into ¼-inch cookies.

Arrange cookies on sheets and bake for 16-20 minutes until edges are golden. Cool on sheets before transferring to wire racks.