Put butter, sugar, salt, caraway and onions in large mixer bowl. Pour water over and stir. Cool to lukewarm, then sprinkle the yeast over and let it sit for a couple of minutes, no more. Yeast should be get a bit foamy. Mix the bread flour and the cocoa. The cocoa doesn't give it a chocolate flavor, but, like the molasses, darkens the color. Put the regular beaters in the mixer and add the flour mix to the water mix. Beat in slowly, then beat on medium for about 5 minutes, to develop the dough. Switch to the dough hooks, and gradually add 2 cups of the rye flour. The dough should be thick and heavy, but not thick enough to knead. Add a little more rye flour if it's too wet. Spoon into a greased loaf pan, put a little butter on your hands, and pat it down lightly to get into it the corners. Bake at 400 degrees for 40 to 50 minutes. Cool completely before slicing.
Some people add a tablespoon of instant coffee, to make the bread even darker, or 2 teaspoons of dried dill. Both are really good. You can also substitute plain yogurt for all or part of the butter. If you don't have bulk yeast, use 2 tablespoons. I buy my yeast at a health food store. It's better yeast, and it's a whole lot cheaper.