Combine discard, water, and flour into a shaggy dough. Rest for 30 minutes.
Add salt, sugar, butter, and yeast. Knead for 5–6 minutes until smooth and elastic.
Let the dough rise until doubled in size (about 1 hour). I used a proofing box at 80 degrees.
Shape into a tight loaf and place in a buttered 8 1/2 x 4 1/2 loaf pan.
Proof a second time until the dough is just above the pan rim (about 1 hour).
Bake at 350°F for 35-45 minutes until golden brown and 190-195 internally.
Remove from pan within 5 mins and cool before slicing.