Sourdough Croissants

Steps

1. 1Feed starter several hours before mixing dough and allow it to peak.
2. 2Sourdough Croissant Dough:
3. 31. Day 1: Mix the dough ingredients until a shaggy dough forms. Remove dough to a clean countertop and knead for a few minutes until a smooth dough forms. Form dough into a ball.
4. 42. Transfer dough to a large bowl and cover. Leave in a warm place (75°-78°F) to ferment for 4 hours, or until it has nearly doubled in size.
5. 53. Cover and refrigerate dough overnight.
6. 6Butter Block:
7. 71. Day 2: Place cold butter on a sheet of parchment paper and sprinkle with the flour. Fold the ends of the parchment loosely to cover the butter and begin pounding the butter with a rolling pin until it begins to flatten. (Update: I find it's easier to measure and fold the parchment before placing the butter inside.)
8. 82. Use a measuring tape to form the parchment paper into a 6x8-inch rectangle pouch over the butter. Flip the pouch over to keep the edges closed. Then use a rolling pin and your fingers to evenly press the butter into the pouch to make a butter block. Chill if butter has begun to melt and looks greasy on the edges. If butter is still cold but very pliable when you bend the package, it is ready for the next step.
9. 9Lock in Butter Block:
10. 101. Remove dough from fridge and turn it out onto a lightly floured surface. Use your hands to begin to shape the dough into a rectangle and pinch the corners to shape them. Roll the dough into a 13x9-inch rectangle.
11. 112. Unwrap butter block and place in the center of your dough with a short side facing you.
12. 123. Bring the ends of the dough over the butter so they meet in the middle of the butter block, encasing the butter. Press every seam to seal so the butter does not escape during rolling. Brush away any loose flour with a pastry brush.
13. 13First Lamination Fold (Book Fold):
14. 141. Turn the dough so the center seam is parallel to you. Use your rolling pin to gently pound the dough to lengthen it. Then roll to a 24x10-inch rectangle. Try to keep the edges and corners straight, and dust underneath with more flour as needed to prevent sticking.
15. 152. Fold the short ends of the rectangle inward to meet in the center (Update: meeting the edges together off-center is actually better). Then fold the new ends together like you're closing a book. Brush away any loose flour as you go.
16. 163. Wrap book in plastic wrap and refrigerate for 20 minutes.
17. 17Second Lamination Fold (Tri-Fold or Letter Fold):
18. 181. Place dough on your lightly floured surface again, this time with the fold facing you. Pound and roll to a 24x10-inch rectangle again.
19. 192. Fold the short ends of the rectangle in thirds this time, as if you are folding a letter. Brush away loose flour as you go.
20. 203. Wrap in plastic again, and chill for 1 hour.
21. 21Shaping and Baking Croissants:
22. 221. Remove dough from fridge and allow it to rest for 20 minutes or until the butter is pliable again. With the folded side facing you, roll to a 18x11.5-inch rectangle. Use a pizza cutter to trim the edges.
23. 232. Cut the rectangle into 5 equal rectangles, measuring for accuracy. Cut each rectangle in half on the diagonal to form 10 triangles.
24. 243. Set a triangle in front of you with the bottom two corners facing you. Brush away loose flour. Gently stretch these corners wider with your fingers and then roll the croissant tight enough that there won't be gaps in the finished pastry. (Update: I now make a small cut in the center of the base of each triangle instead of stretching the edges, then roll.) Lightly press the tip of the triangle into the croissant to help it adhere to the underneath side. Repeat for remaining triangles.
25. 254. Arrange the croissants on 2 parchment-lined half sheet pans, 5 to each pan. Cover with plastic wrap (not too tightly) and leave at room temperature to proof for several hours (do not exceed 75°F during proofing), or until croissants have doubled in size and wobble like marshmallows when you bump the pan. You should begin to see some separation in the layers when proofing is finished. Meanwhile, preheat your oven to 400°F.
26. 265. Beat your egg and milk together to make an egg wash. Use a pastry brush to brush egg wash over each proofed croissant, avoiding the laminated edges.
27. 276. Bake one pan at a time on the center rack for 15-20 minutes, or until deeply golden brown. (Hold second pan in the fridge until you're ready to egg wash and bake if done proofing.)
28. 287. Place croissants on a cooling rack, and serve or store once they've cooled completely.