Make the Dough: In a bowl, mix flour, sugar, yeast, and salt. Add warm milk and knead until smooth. Cover and chill for 4 hours.
Prepare the Butter Block: Flatten cold butter into a 7-inch square between parchment paper. Chill.
Laminate the Dough: Roll out the dough into a 10x14-inch rectangle. Place the butter block in the center, fold the dough over, and seal. Roll out again, fold into thirds, and chill for 1 hour. Repeat this process twice.
Shape the Croissants: Roll the dough into a large rectangle, cut into triangles, and roll tightly from the base to form croissants.
Proofing: Place croissants on a baking tray, cover, and let rise for 2-3 hours until doubled in size.
Egg Wash & Bake: Preheat oven to 375°F (190°C). Mix egg with water and brush over croissants. Bake for 18-20 minutes until golden brown.
Cool & Enjoy: Let cool before serving. Best enjoyed fresh!