In a small bowl, combine yeast and sugar with warm water. Stir to dissolve and let stand until foamy, about 10 minutes.

Heat the molasses and butter in a small bowl, until melted and smooth. Add the vinegar, and let the mixture cool slightly to around 100F.

In a large bowl, combine rye flour, 1 ½ c all purpose flour, bran, cocoa powder, ground espresso, caraway, fennel and salt. Mix to combine.

Add yeast and molasses mixtures to the flour mixture. Mix until smooth.

Add additional all purpose flour as needed to bring the dough together. It will be very sticky but firm

Turn the dough out onto a lightly floured surface and knead for 7-10 minutes to create a firm, but springy dough. (Alternately, combine the ingredients in the bowl of your stand mixer and knead with the dough hook on low speed for 3-5 minutes.)

Shape the dough into a ball and place it in a greased bowl. Cover the bowl with a damp tea-towel and let it rise in a warm, draft-free place until doubled, about 1 ½ to 2 hours.

Turn the dough out onto a lightly floured surface and shape it into a round. Place it directly on baking sheet dusted with flour. Cover loosely with a damp tea towel. Let rise until nearly doubled in size roughly 1 hour.

Near the end of your rising time, preheat the oven to 425F.

Uncover the dough sprinkle it with a dusting of flour and some caraway seeds (if desired). Using a serrated knife, quickly cut an 'X', roughly 1 ½ inches deep across the top of the dough.

Bake for 20 minutes at 425F.

Reduce the heat to 350F, and bake for another 20-25 minutes, or until the loaf develops a deep color and sounds hollow when you tap on it.

Remove the bread from the oven and let it cool for at least 15 minutes on a rack before slicing into it.