Measure the sourdough starter into a large bowl, and then whisk in 100 grams warm water. Stir in 150 grams light rye flour, to form a shaggy, loose dough. Cover the bowl with a tight-fitting lid or square of plastic wrap, and then let it sit at room temperature at least 12 and up to 16 hours, or until proofed and bubbly.
Make the dough.
Scrape the levain into the basin of a stand mixer, and then dump in 150 grams dark rye flour and 150 grams light rye flour. Add the water, molasses, salt, and caraway seeds. Using the dough hook, knead the dough about 5 minutes, and then cover the bowl and let the dough rise about 3 hours, or until puffy.
Generously dust your banneton with flour.
Turn the dough out onto a floured surface, and then form it into a boule. Place it seam-side up into the floured banneton, and then cover it lightly with a tea towel. Allow it to rise about an hour. Then transfer it to the fridge.
Baking the bread.
Place a Dutch oven into the oven, and then preheat the oven to 500 F.
Once the oven reaches temperature, turn down the heat to 450 F. Then turn the dough out onto a piece of parchment paper, and score the loaf.
Remove the preheated Dutch oven from the oven, and then carefully lift its top. Gently lower the dough into the Dutch oven, cover it, and then return the pot to the oven. Bake for 30 minutes, and then uncover the Dutch oven and continue baking a further 15 minutes.
Transfer the bread to a wire rack and allow it to cool completely before slicing and serving.