Mix all wet ingredients (excluding butter), then add the dry. Knead by hand or with a stand mixer for 5 minutes until smooth. Cover and rest 1 hour. Mix in butter.
Do 4 sets of stretch & folds every 30 minutes, then bulk ferment until doubled. Cold proof overnight.
The next day, shape into a batard-style log, seal the edges, and place in a generously buttered loaf pan. (Double the recipe and use a Pullman pan for a classic Pain de Mie shape.)
Final proof until the dough just peeks above the pan. Preheat oven to 420°F. Brush the top with an egg yolk + 1 tsp milk mixture.
Bake 30 min at 420°F, then reduce to 375°F and bake 15 more minutes. Carefully remove, cool on a wire rack, slice, and enjoy that perfect soft crumb 🍞
