How to Make It
Whisk together 2 1/2 cups warm water, yeast, and 1 tablespoon sugar in a medium bowl or large measuring cup until dissolved. Let mixture stand until foamy, about 15 minutes. Meanwhile, whisk together flour, salt, and remaining 3/4 cup plus 2 tablespoons sugar in a large bowl. Pour yeast mixture into a very large bowl, and add flour mixture; stir with a wooden spoon until a shaggy dough forms, about 1 minute.
Transfer dough to a very lightly floured work surface (use just enough flour to keep dough from sticking to fingers and work surface), and knead until smooth, about 10 minutes. Pinch off a small piece of dough, and, using fingers, gently stretch until dough piece is almost translucent without tearing. If dough tears, continue to knead, 2 to 3 minutes, checking after 2 minutes to test dough before kneading an additional 1 minute. Lightly grease a large bowl with 1/2 tablespoon butter; place dough in greased bowl. Cover with plastic wrap, and let rise at warm room temperature (about 75°F), free from drafts, until doubled in size, 1 hour to 1 hour and 30 minutes.
Using hands, spread 3 tablespoons butter in a thin, even layer on a clean work surface into a 2-foot square. Place proofed dough in center of butter square. Grease hands with 1/2 tablespoon butter to avoid tearing dough. Pat proofed dough into a flat disk. Working from center of dough disk with buttered hands, gently spread and push dough outwards using circular motions to form a thin, even 2-foot square. (Dough should form a thin sheet on top of the 2-foot-square butter layer.) Using hands, spread 3 tablespoons butter into a thin, even layer on top of the dough square.
Preheat oven to 250°F. Grease the inside of a 24- to 26-centimeter jachnun pan or a 10- to 11-inch 5-quart lidded oven-safe saucepan or stockpot with remaining 1/2 tablespoon butter; set aside. Roll up buttered dough square into a tight 2-foot-long log; coil log to form a tight spiral, with even rings. Carefully transfer dough spiral into prepared pot, seam side down. Cover with lid, and let rise at warm room temperature (about 75°F) until almost doubled in size, about 1 hour. Before baking, ensure lid is fully sealed. If using stockpot, wrap entire pot and lid in aluminum foil to ensure pot is airtight.
Bake in preheated oven until bread is deep golden brown with the aroma of molasses, about 8 hours (avoid opening oven or lid while baking). Uncover and transfer pot to a wire rack, and let bread cool in pan 30 minutes. Remove bread from pot, and let cool completely on wire rack, about 1 hour and 30 minutes.
For the best results, bake the bread in a sealed thin aluminum tin (jachnun pan) or stainless pot so it heats evenly while baking. Avoid using a thick metal pan (such as a Dutch oven) or covered casserole dish. Purchase a jachnun pan from Amazon or superkacher.com.