A unique black and white cookie, these Marbled Cookies are essentially your favorite marble cake in cookie form. A straightforward butter cookie recipe, melted dark chocolate is added to half of the dough to create the signature marble effect. Make the cookie dough at least the day before you plan on baking the cookies as the dough needs to refrigerate overnight. Roll the dough into a cylinder before chilling so that you can just slice and bake the cookies the next day.