In a large bowl, add the all purpose flour, unsalted butter, and boiling water. Let it sit for a few seconds to melt the butter, then stir until thick and pasty.
Work all the melted butter into the paste so it’s evenly mixed.
Add sugar (or honey), buttermilk, egg, and sourdough starter. Stir with a whisk until well mixed.
Add the remaining all purpose flour and salt. Stir until too difficult to mix with a fork, then use your hands to bring the ingredients together into a cohesive, smooth dough.
Grease the bowl and cover with a non-permeable covering. Rise at room temp for 12 hours, or until double.
On a floured surface, gently stretch and pat the dough into a rectangle about the width of your loaf pan. Shape the loaf as preferred.
Place the loaf in a greased loaf pan. Cover and rise for 2 hours or until double.
Bake on the center rack of a preheated 375F oven for 50 minutes, or until the internal temperature is 195F / 91C.
Cool fully before slicing, ideally 8 hours.
