Flaky, buttery, and beautifully laminated. A weekend project that produces bakery-quality croissants with incredible sourdough depth.

DAY 1 - DOUGH: Combine the starter, cold milk, sugar, salt, and flour. Mix until a dough forms. Knead for 5 minutes — it should be smooth but not overly developed.
Shape into a rectangle, wrap in plastic, and refrigerate overnight (8-12 hours).
DAY 1 - BUTTER BLOCK: Pound the cold butter with a rolling pin into a 6x8 inch rectangle between two sheets of parchment. Mix in the flour while pounding. Refrigerate.
DAY 2 - LAMINATION: Roll the dough into a 12x8 inch rectangle. Place the butter block on one half and fold the dough over it, sealing the edges. This is the lock-in.
Roll the dough out into a long rectangle, about 8x20 inches. Perform a letter fold (fold the bottom third up, top third down). Wrap and refrigerate for 1 hour.
Repeat the roll-out and letter fold two more times, chilling for 1 hour between each fold. You now have 27 layers of butter.
After the final fold and rest, roll the dough into a large rectangle about 1/4 inch thick.
Using a sharp knife or pizza cutter, cut into long triangles (base about 4 inches).
Starting from the base, roll each triangle up toward the point, gently stretching as you go. Curve the ends inward to form a crescent shape.
Place on a parchment-lined baking sheet. Cover loosely and let proof at cool room temperature (68-72°F) for 2-3 hours until visibly puffy and jiggly when the pan is tapped.
Preheat oven to 400°F (200°C). Brush with egg wash.
Bake for 18-22 minutes until deep golden brown and flaky. Let cool on the pan for 10 minutes.