This pillowy soft, subtly sweet sandwich bread is a beloved breakfast staple in Japan and is typically eaten sliced very thick, lightly toasted, and served with accompaniments like butter and jam.
Whisk together milk, 1/2 cup water, and flour in a small saucepan until smooth. Cook over medium-low heat, whisking frequently, until thickened slightly but still pourable (it should have the consistency of loose pudding), 3 to 4 minutes. Transfer to a small bowl; let cool until warm to the touch but no longer hot, about 10 minutes.
In a large bowl, whisk together flour, milk powder, sugar, salt, and yeast. Make a well in center of mixture. Add milk, melted butter, and starter to well; stir until a dough forms. Transfer to a lightly floured work surface and knead until dough is smooth and elastic and springs back when lightly pressed, 8 to 10 minutes. Transfer to a bowl brushed with butter; brush top of dough with more butter. Cover bowl with plastic wrap and let rise until doubled in volume, 45 minutes to 1 hour.
Preheat oven to 375 degrees. Brush two standard 9-by-5-inch loaf pans with butter. Punch down dough. Transfer to a clean work surface and divide in half. Roll each half into an approximately 9-inch log; transfer to prepared pans. Loosely cover with plastic wrap and let rise until more than doubled in volume (doughs should rise about 1 1/2 inches above tops of pans), 45 minutes to 1 hour.
Whisk egg white with 1 teaspoon water and gently brush onto tops of dough. Bake until puffed, golden brown, and a thermometer inserted in centers registers 200 degrees, 35 to 40 minutes. Let cool in pans on a wire rack 15 minutes. Flip loaves out onto rack; let cool completely before slicing and serving, or store, unsliced and wrapped in parchment-lined foil at room temperature up to 3 days, or sliced and frozen up to 3 months.
Once sliced and frozen, this bread can be toasted directly from the freezer.