Combine flour, starter, milk, eggs, egg yolks, and sugar. Stir to a shaggy dough, cover, and rest 30 minutes.

Add salt and knead with a dough hook on low for 2 minutes. Add soft butter a tablespoon at a time on low, then knead on medium 8 to 10 minutes until smooth and passing windowpane test.

Shape into a ball, place in a greased bowl, cover, and rise 4 to 6 hours at room temp. Perform coil folds every 45 minutes for the first 3 hours, then let rise until doubled.

Turn dough onto a lightly floured surface, pat into a 9x12-inch rectangle, fold short sides in, roll tightly, and place seam-side down in a greased 9x5-inch loaf pan.

Cover and proof until the dough crowns 1 to 1 ½ inches above the pan rim, 3 to 5 hours. Look for a slow spring back when poked.

Preheat oven to 375°F (190°C). Brush with egg wash, bake 35 to 40 minutes until deep golden and internal temp reads 190°F (88°C). Cool 5 minutes in pan, turn out, brush with melted butter, and slice warm.