Mix the sourdough starter, bread flour, and water in an empty jar. Cover and set in a warm location for about five hours until doubled in size, ripe, and bubbly.
Once the levain is ripe, make the croissant dough by adding the listed ingredients to a stand mixer and mixing until the dough is tacky.
Transfer the dough to a bowl, cover, and place in a warm location to proof for about 5 hours or until the dough doubles in size.
Prepare the butter block and refrigerate it.
After an overnight proof, seal the butter block inside the croissant dough and refrigerate.
Perform three simple folds on the dough and refrigerate after each fold.
Shape the croissants and allow them to proof until they've doubled in size.
Preheat the oven, apply egg wash on the croissants, bake them, and enjoy the flaky sourdough croissants.
