In a large bowl, mix butter and sugar until creamy.
Add flour and optionally vanilla and mix well.
Divide the dough in half, wrap one half in plastic wrap and refrigerate.
Mix the remaining half of the dough with about 1 Tbsp cacao.
Add another Tbsp of cacao to 1/3 of the cacao dough to obtain a small piece of dough with a darker brown colour. Adjust the amounts of the cacao powder to obtain a smaller piece of dough with a dark brown and a somewhat larger piece of dough with a lighter brown colour.
Wrap each of the brown doughs in plastic wrap and refrigerate for about 1 hour.
When the dough has cooled and can be handled, divide each piece of dough into 6 equal parts. You should have 18 pieces of dough then.
Roll each piece of dough into a cylinder about 10-12 inches (25-30 cm) in length.
Roll out each piece of the dark brown dough into a rectangle and wrap the dark brown rectangles around the light brown cylinders, leaving a gap (i.e. the two sides of the dark brown dough should not meet each other when wrapped around the light brown cylinder).
Now roll each piece of the vanilla dough into a rectangle and wrap these around the dark cylinders.
Arrange your 6 small, 3-coloured rolls to make one large roll. Flatten the sides, wrap in plastic wrap and refrigerate for at least two hours.
Baking the cookies
Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
Slice the dough, place cookies on the prepared baking sheet and bake for about 8-15 minutes (depending on the thickness of your cookie slices), or until light brown.
Cool for about 5 minutes on the sheet, then remove to racks and cool completely.