Autolyse: Mix bread flour, whole wheat flour, and milk in a large bowl. Cover and rest at room temperature for 24 hours.
Add sourdough starter, sugar, and salt: Mix until just combined.
Add softened butter (or cream cheese): Mix until dough comes together in a shaggy mass.
Knead: Use a stand mixer with a dough hook attachment or knead by hand for 10 minutes.
Bulk fermentation: Place dough in a lightly oiled bowl, cover, and let rest at room temperature for 4-5 hours, or until doubled in size.
Shape: Gently deflate dough and shape into a round or oblong loaf.
Bake: Preheat oven to 375°F (190°C). Place dough onto a preheated baking stone or baking sheet. Bake for 35-40 minutes, or until golden brown.
Cool: Let cool on a wire rack for at least 1 hour before slicing.