An incredibly rich, buttery bread with a tender crumb. Uses sourdough starter for added depth of flavor.

In a stand mixer with the dough hook, combine the starter, eggs, milk, flour, sugar, and salt.
Mix on low for 2 minutes until combined, then increase to medium and knead for 8 minutes until the dough is smooth and pulls away from the sides.
With the mixer running on medium-low, add the butter one cube at a time, waiting for each piece to incorporate before adding the next. This takes about 10 minutes.
Once all butter is incorporated, increase speed to medium-high and knead for another 5 minutes. The dough should be very smooth, shiny, and slap against the sides of the bowl.
Transfer to a greased bowl, cover, and let rise at room temperature for 2-3 hours.
Punch down, cover tightly, and refrigerate overnight (or up to 48 hours).
The next day, divide the chilled dough into 8 equal pieces. Shape each into a tight ball.
Place in a greased 9x5 loaf pan (in two rows of 4). Cover and let rise for 2-3 hours until puffy and dough crowns above the pan.
Preheat oven to 350°F (175°C). Brush with egg wash.
Bake for 30-35 minutes until deep golden brown. Tent with foil if browning too quickly.
Let cool in pan for 15 minutes, then turn out onto a wire rack.