Unsweetened chocolate, molasses, and ground caraway seeds give this bread its unmistakable depth of flavor.
Brush a large bowl with butter; set aside. Combine 1/4 cup warm water with sugar; sprinkle yeast over top. Set aside. In a small saucepan, heat espresso, chocolate, molasses, and 1 1/2 cups water over medium. Cook, stirring often, just until chocolate is melted, about 5 minutes. Remove from heat and let cool until warm but not hot (below 110 degrees).
In the bowl of a stand mixer fitted with a dough hook attachment, combine both flours, caraway, and salt. Add the chocolate mixture then yeast mixture. Mix by hand, using the hook, to combine. Turn mixer on medium and mix until dough comes together in a smooth, springy ball around the hook, 12 to 15 minutes. Remove a small piece of dough and stretch it. If the dough holds together in a thin "windowpane" without breaking, the gluten is sufficiently developed. Add butter, 1 tablespoon at a time, until incorporated, about 4 minutes. Turn out onto a well-floured surface and knead a few times to form into a ball. The dough will be slightly sticky; don't be tempted to add more flour. Transfer to buttered bowl, turning once to coat, and cover loosely with buttered plastic. Let sit at warm room temperature until doubled in size, about 1 1/2 hours.
When dough has doubled, turn out onto a work surface. Pat dough out into a rectangle, roughly 10 by 12 inches with short side facing you. Fold bottom edge two-thirds of the way up the rectangle, and top edge down to cover. Rotate dough 90 degrees and repeat the fold. (This is called a business-letter turn.) Return to buttered bowl and top with buttered plastic. Let sit at warm room temperature until doubled in size again, about 1 hour. Remove from bowl, gently deflate, and work into a round boule about 6 inches in diameter. Place a clean dish towel in a medium bowl and generously dust with flour. Place boule, seam-side up, in bowl. Loosely fold floured towel ends over dough to cover. Let sit at warm room temperature until doubled, 30 to 45 minutes.
Meanwhile, preheat oven to 450 degrees and place a 9- or 9 1/2-inch enameled cast-iron pot in oven with the lid to preheat. When dough is ready, carefully remove the lid, pull back corners of towel, and flip bowl upside down to transfer bread to the heated pot, seam-side down. Quickly slash an "X" in the top of the bread with a serrated knife and using an oven mitt, cover with lid. Return pot to oven and bake 20 minutes. Remove lid and continue to bake until bread feels hollow when tapped and internal temperature registers 205 degrees, 20 to 30 minutes more. Immediately turn bread out of pot onto a cooling rack. Let cool at least 2 hours before slicing.