Mix the active sourdough starter with flour and lukewarm water in a small bowl, stirring until no dry flour remains. Cover loosely and let sit at room temperature (around 78-80°F) for 10-12 hours until the mixture is bubbly, doubled in volume, and shows signs of fermentation. This creates the levain—the living culture that will leaven your bread and give it that distinctive sourdough flavor. I've found that using filtered water helps prevent any chlorine from interfering with fermentation.

In a large bowl, combine the levain from Step 1 with bread flour, water, salt, and granulated sugar. Mix with your hands or a wooden spoon until all flour is hydrated and a shaggy dough forms—there should be no dry flour visible, but the dough doesn't need to be fully smooth yet. Cover the bowl and let it rest for 30 minutes at room temperature. This autolyse period allows the flour to fully absorb water and begins gluten development before you start folding.

Grate the cold butter into thin shards using a box grater or food processor—cold butter distributes more evenly and creates the lamination essential for croissant texture. Perform a series of gentle stretching folds on the dough, sprinkling the first half of the grated butter across the surface as you fold, then rest for 30 minutes. Repeat the folding process with the remaining butter, incorporating it fully through another set of folds, and rest another 30 minutes. Perform one final set of folds without any additions to build tension. The gradual incorporation of butter creates thin, distinct layers that give sourdough croissant its signature flaky structure. I recommend working with the dough in the bowl rather than on the counter—it's less messy and you keep everything contained.

Cover the dough from Step 3 and let it rise at room temperature for 6-7 hours. During this time, the dough will increase in volume and develop flavor from the long, slow fermentation. No additional stretching or folding is needed during bulk fermentation—the gluten is already well-developed from Step 3, and the dough just needs time to rise gently.

Turn the dough from Step 4 onto a lightly floured work surface and gently pre-shape it into a round, then let it rest for 30 minutes. This resting period relaxes the gluten. After 30 minutes, perform a final shaping by gently pulling the dough toward you to build surface tension, then place it seam-side up in a floured bowl or banneton basket. Cover and refrigerate for 12-18 hours. The cold proof is crucial—it stops fermentation, develops more sour flavor, and makes the dough easier to score and bake. The extended chill also helps the laminated layers set properly, giving you better oven spring and flakiness.

Preheat your oven and a Dutch oven (or covered baking vessel) to 450°F for at least 20 minutes. Remove the chilled dough from the refrigerator and flip it onto a piece of parchment paper, then score the top with a sharp blade using one confident slash or your preferred pattern. Carefully place the dough (on parchment) into the preheated Dutch oven, lower the heat to 425°F, cover with the lid, and bake for 25 minutes. The trapped steam in the covered pot allows the crust to expand and brown beautifully. Remove the lid and bake for another 20 minutes until the crust is deep golden brown and the internal temperature reaches 195-200°F when measured with a thermometer in the center. Let the finished bread cool on a wire rack for at least 1 hour before slicing—this allows the interior crumb to set properly and makes for cleaner slices.