Begin with the vanilla dough. In a large bowl, cream together the butter and both sugars until light and fluffy, about 3-5 minutes. Beat in the egg yolk and vanilla extract until fully combined. Add the flour and salt and mix until just combined. Place the dough in a bowl or on a clean counter while you prepare the chocolate dough.

In a large bowl, cream together the butter and both sugars until light and fluffy, about 3-5 minutes. Beat in the egg yolk, cooled chocolate, and vanilla extract until fully combined. Add the flour, cocoa powder and salt and mix until just combined. Place the dough in a separate bowl. Cover in plastic wrap and refrigerate until firm, about 1-2 hours.

Once the dough has firmed up, use your fingers to roll about 2 tablespoons of vanilla dough into a long snake about 8" long. It helps to do this on a flat, non-stick surface. Repeat until you have three rolls of vanilla dough and three rolls of chocolate. Stack the smaller rolls on top of each other, creating one thick roll, and twist them together carefully - you don’t want to break the dough.

You should have one thick, twisted log of dough at this point, with swirls of the chocolate and vanilla dough together. Roll the dough gently to smooth out the sides, being careful not to make the log too thin - it should be about 2” in diameter. Wrap the log in plastic wrap and store in the refrigerator for at least one hour or up to 2 weeks. Repeat with the remaining dough until all has been used.

Preheat oven to 325°F and line a large baking sheet with parchment paper. Slice cookies 1/4” thick and place 1 inch apart on the baking sheet. Bake for 12-15 minutes or until lightly browned around the edges.

The dough can be frozen for up to 6 months.