Step 1: Prepare the Dough

1. In a mixing bowl, combine sourdough starter, milk, sugar, and salt.
2. Gradually add flour, mixing until a soft dough forms.
3. Knead for 8-10 minutes, until smooth and elastic.
4. Cover and let rest for 2-4 hours at room temperature, until slightly puffed.

Step 2: Prepare the Butter Block

1. Place cold butter between two sheets of parchment paper.
2. Roll into a thin rectangle and chill in the fridge.

Step 3: Laminate the Dough (Butter Layering)

1. Roll out the dough into a large rectangle.
2. Place the butter block in the center and fold the dough over it.
3. Roll out and fold into thirds (like folding a letter).
4. Chill for 30 minutes, then repeat the rolling and folding three more times.

Step 4: Shape & Proof

1. Roll out the laminated dough and shape into a loaf.
2. Place into a greased loaf pan and cover.
3. Let rise at room temperature for 8-12 hours (overnight is best).

Step 5: Bake & Serve

1. Preheat oven to 375°F (190°C).
2. Brush with egg wash (if using).
3. Bake for 30-35 minutes, until golden brown.
4. Let cool for 10 minutes, then slice and enjoy warm!