Combine flour, sugar, salt, yeast, milk, water, and melted butter. Knead until smooth, shape into a rectangle, and chill overnight.
Pound the 350g of cold butter into a 6x8-inch rectangle. Chill until pliable but firm.
Encase the butter block in the chilled dough. Perform three single folds (letter folds), chilling the dough for 30 minutes between each turn.
Roll the laminated dough out, cut into strips, braid or roll into a cylinder, and place in a buttered 9x5-inch loaf pan.
Proof in a warm spot until doubled and jiggly. Brush with egg wash and bake at 400°F (200°C) for 15 minutes, then at 375°F (190°C) for 25-30 minutes until deep golden brown.
