Mix 150 g active sourdough starter, 25 g honey, 150 g water, and 200 g milk. Add 570 g bread flour and 100 g whole wheat flour and mix to a shaggy dough. Rest 30 minutes.
Add 15 g sea salt and 25 g water. Mix to incorporate.
Add 80 g butter gradually, mixing for about 10 minutes, until dough is smooth and elastic.
Bulk ferment until doubled (mine took ~7 hrs in a warm spot).
Divide and shape into sandwich loaves. To shape, gently flatten each piece into a rectangle to press out large air bubbles. Fold the dough in thirds like a letter, then roll it up tightly from the short end. Tuck the edges under and create surface tension as you seal the seam.
Grease your loaf pans with butter or a light spray of oil. Place the shaped dough inside and let it rise for 2–3 hours, or until puffy and just cresting over the rim of the pan.
Bake at 375°F for 35 minutes.
Remove from loaf pans and set on cooling rack to prevent soggy bottoms.
Brush with melted butter while still warm for extra softness!