Ultra-flaky, buttery croissants made with naturally leavened dough for a delicate tang and tender layered crumb.
Make the dough: In a stand mixer, combine flour, sugar, salt, whole milk, and active sourdough starter. Knead on medium/low until the dough pulls away from the bowl and is glossy and stretchy, about 10 minutes.
Bulk ferment: Transfer to a lightly greased bowl, cover tightly, and rest at room temperature overnight (about 8 hours).
Chill and pre-roll: Chill dough 1 hour, then roll to a 10-by-16-inch rectangle. Chill 4 hours or overnight.
Prepare butter block: On parchment, form butter into an 8-by-10-inch rectangle (enclosed in the parchment) and roll to an even thickness. Chill 15–30 minutes until the butter consistency matches the dough (pliable, not hard).
Encase butter: Peel butter from parchment and place centered on the dough. Fold dough over to fully encase the butter and pinch seams closed.
Laminate – turn 1: Roll to 10-by-16 inches. If the dough resists, chill to relax gluten. Fold in thirds (like a letter).
Laminate – turn 2: Tap gently to keep butter pliable, roll again to 10-by-16 inches, and fold in thirds. If warming, chill 30 minutes before continuing.
Laminate – turn 3: Roll once more to 10-by-16 inches and fold in thirds (3 total turns). Chill the laminated dough 4 hours.
Shape: Roll to a 10-by-20-inch rectangle. Mark every 4 inches along one long side; on the opposite long side, mark every 4 inches offset between the first marks. Cut diagonally between marks to form long triangles.
Roll: Starting at the wide end, roll each triangle tightly into a crescent. Proof at room temperature until puffy, about 2 hours. Optionally chill shaped croissants 1 hour to help them hold shape.
Bake: Preheat oven to 375°F (190°C). Brush with egg yolk wash. Bake 30 minutes, until deeply golden.