Preheat oven to 200°. Grease two 5" × 81⁄2" loaf pans with butter and set them aside in a warm spot. Put yeast into a small glass bowl, add treacle and 1⁄2 cup lukewarm water, and stir to dissolve. Set aside and let rest until yeast bubbles and becomes frothy, about 10 minutes.
Put flour and salt into a large ovenproof bowl and stir well to combine. Place bowl in oven and let rest until flour mixture is warmed through, about 10 minutes. Remove bowl from oven, add the yeast mixture and 3 1⁄2 cups lukewarm water, and mix together with your hands until well combined and a sticky dough forms.
Increase heat to 400°. Divide dough evenly between the 2 prepared loaf pans, cover with a clean kitchen towel, and let rise in a warm spot until bread has grown by one-third, 15–20 minutes. Bake bread on middle rack of oven until the loaves are browned on top, about 45 minutes. Loosely cover loaves with foil, then continue to bake for 25–30 minutes more.
Let bread cool in pans for 10 minutes, then gently run a table knife around inside edges of pans to loosen. Turn loaves out onto a rack and let rest until completely cool, 2–3 hours.