Mix all ingredients except the black cocoa powder in a stand mixer until combined and knead for 5 minutes. Add to a well-oiled bag and refrigerate for up to a week. Bring dough to room temperature either naturally or in warm water. Separate dough into two pieces, I made the darker portion a slightly smaller piece. Add the cocoa powder to the smaller piece. Make both pieces into a rectangle, with the darker one being slightly smaller. Place the darker dough on the lighter dough, and roll into a log. Place in 9 inch bread pan and allow to rise until dough is just at the top (this could take 2 hours, this dough rises very slowly even in a warm place). Cut as deeply as you can with a good bread lame and cover with foil. Bake in preheated 375 degree Fahrenheit oven for 25 minutes and remove the foil. Bake for another 10 minutes, or until internal temperature reads 190. Rub top with butter while still warm.