# Steps:
01 - In a large bowl, combine bread flour, sugar, and salt. Add cold milk, sourdough starter, and softened butter. Mix until just combined, then knead for approximately 8 minutes until a smooth, elastic dough forms.
02 - Shape dough into a rectangle, wrap tightly in plastic wrap, and refrigerate for at least 2 hours.
03 - Place 1 cup plus 1 tablespoon cold butter between two sheets of parchment paper. Pound and roll into an 8 x 6 inch rectangle. Chill until firm but still pliable.
04 - On a lightly floured surface, roll out dough to a 16 x 8 inch rectangle. Position butter block on one half, fold dough over the butter, and seal all edges firmly.
05 - Rotate dough 90 degrees and roll into a 24 x 8 inch rectangle. Fold into thirds like a business letter, wrap in plastic, and refrigerate for 1 hour.
06 - Repeat the rolling and letter-fold process two more times, chilling for 1 hour between each turn to maintain dough temperature and butter distribution.
07 - After the final fold, wrap dough in plastic and refrigerate overnight for 8 to 12 hours to develop flavor and ensure proper lamination.
08 - The next morning, roll chilled dough to a 24 x 12 inch rectangle, approximately 1/5 inch thick. Cut into 12 long triangles with equal dimensions.
09 - Beginning from the wide end of each triangle, roll tightly toward the point to form a crescent shape, tucking the pointed tip underneath. Place shaped croissants on parchment-lined baking sheets.
10 - Cover croissants loosely with plastic wrap and proof at room temperature for 2 to 3 hours until they have doubled in size and feel light when touched.
11 - Preheat oven to 400°F. Whisk together egg and milk. Brush the egg wash lightly over each croissant using a pastry brush.
12 - Bake for 18 to 22 minutes until golden brown with a crisp, flaky exterior. Allow croissants to cool slightly on the baking sheet before serving.