To make the chocolate peppermint dough, cream butter, powdered sugar and peppermint extract in the same bowl (don’t worry about washing it). Add cocoa powder and flour and mix on low speed until dough begins to clump together, about 1 minute. It will be crumbly but will stick together when squeezed. Transfer chocolate peppermint dough to a work surface, shape into a 1-inch thick square disk and wrap in plastic wrap. Refrigerate until chilled, about 30 minutes.

Roll out vanilla dough in between two pieces of parchment paper to form a 9-by-12 inch rectangle. Set aside and repeat with chocolate peppermint dough (it will be much thinner). Tear off random, irregular sized pieces of chocolate dough and place them on top of vanilla with small gaps between each piece; it will look like the spots of a giraffe. When the surface is covered (you won’t use all of the chocolate dough yet), fold the dough in half lengthwise. Roll out again to a rough 9-by-12 inch rectangle, tear remaining chocolate dough into irregular shapes and place randomly on the surface. Fold dough in half lengthwise, press to flatten, then fold in half again to form a rough log. Twist the log as if to ring out a towel, squishing the dough together if it starts to crack. The more twisting you do, the more marbled the cookies will be. Squeeze the log to make sure there are no cracks in the center, then roll the dough firmly on the counter until smooth. Wrap in plastic wrap. If desired, cut a lengthwise slit in an empty paper towel roll, place the log inside and secure with a few rubber bands. This will keep the log round while it chills. Refrigerate for at least 30 minutes, until very firm.

When ready to bake the cookies, preheat oven to 350°F.

Slice the dough into 1/4” thick slices and arrange on a parchment paper lined baking sheet. The cookies don’t spread much and can be fairly close together. Bake for 16- 20 minutes, depending on the thickness, or until the vanilla dough is slightly golden. Let sit on sheet for a few minutes before transferring to a rack to cool completely.