Mix the rye, water, yeast, and molasses in a very large stainless-steel container. Cover with a plate and let sit 3 days in a cool spot, i.e. about 60 degrees, so that the starter can ferment slowly.
Add the water and malt powder to the starter and dissolve using a whisk or an ancient Bavarian wooden spoon, if you're lucky enough to have one!
When the starter is ready, place the remaining ingredients in a large bowl. Make a well in the middle and add the starter. Knead in the bowl for at least 10 minutes or until the bread starts forming a nice large ball. If it doesn't seem to come together as quickly as you want, let it rest for a few minutes and you'll be able to knead more easily. The dough should be stiff.
Keep the dough in the bowl and place inside a large white garbage bag so that the dough will not dry out. Leave it to rise for 10 to 12 hours in the winter and 5 to 6 hours in the summer.
Punch the dough down, turn it out on a big work surface, and divide into 4 big portions. Shape into circles or place in greased bread pans. Dust with cornmeal and cover with a plastic bag again. Let it rise until doubled.
With a paring knife make 6 slashes in any pattern you like.
Place in a cold oven with a broiler pan with hot water on the bottom. Turn the oven to 350 degrees and bake for about 1 1/2 hours or until the loaves are done, sounding hollow when tapped with a knife.