Stir in flour a little at a time until you cannot mix it together anymore. Dump out onto a flat surface and knead until smooth, about 5 minutes. Place in a large bowl covered with a dish towel and let rise until doubled in size. Grease 9 x 5 loaf pans. Punch down dough, divide in half and shape into a loaf.
Place loaves into pans, cover with dish towel and let rise until it is double in size.
Butter top of loaves and bake in a preheated 350 F oven for 1 hour. Let cool 10 minutes before removing from pans. Place chocolates and butter in a bowl over a pot of simmering water to melt. Let cool to room temperature. Whisk together egg yolks, sugar and coffee until well combined.