# Method:
01 - In a large bowl, combine bread flour, granulated sugar, and fine sea salt. Add cold milk, sourdough starter, and softened butter. Mix until just combined, then knead until a smooth, elastic dough forms, approximately 8 minutes.
02 - Shape dough into a rectangle, wrap in plastic wrap, and refrigerate for at least 2 hours.
03 - Place cold butter between two sheets of parchment paper. Pound and roll into an 8 by 6 inch rectangle. Chill until firm but pliable.
04 - On a lightly floured surface, roll out dough to a 16 by 8 inch rectangle. Place butter block on one half, fold dough over, and seal edges with a rolling pin.
05 - Turn dough 90 degrees and roll into a 24 by 8 inch rectangle. Fold into thirds in a letter fold pattern, wrap in plastic, and chill for 1 hour.
06 - Repeat the rolling and folding process two additional times, chilling for 1 hour between each turn.
07 - After the final fold, wrap dough in plastic and refrigerate overnight, 8 to 12 hours.
08 - Roll dough to a 24 by 12 inch rectangle, approximately 1/4 inch thick. Cut into 12 long triangles using a sharp knife.
09 - Starting from the wide end of each triangle, tightly roll toward the point into a crescent shape, tucking the tip underneath. Arrange on parchment-lined baking sheets.
10 - Cover loosely with a kitchen towel and proof at room temperature until doubled in size, approximately 2 to 3 hours.
11 - Preheat oven to 400°F.
12 - Whisk together egg and milk. Brush croissants lightly with egg wash mixture.
13 - Bake for 18 to 22 minutes until golden brown and crisp. Cool slightly before serving.