Mix 10 grams of ripe sourdough starter with 100 grams of water and 100 grams of flour. Cover and let sit overnight at 78ºF until doubled in size, bubbly, and active.

To the bowl of a stand mixer fitted with a dough hook, add ripe levain, water, milk, granulated sugar, melted butter, salt, and bread flour. Knead on low to medium speed for about 8-10 minutes until you have a smooth, cohesive, and strong dough.

Place the dough in a clear container or glass bowl, cover with plastic wrap and let it rise in a warm spot until almost doubled in size, about 6 hours at (78–80ºF).

Prepare the butter block. Tear off a 28-inch sheet of parchment paper and fold it to create a 9x12-inch rectangle pocket. Slice the 340 grams of butter into six equal rectangular pieces and arrange them in a line inside the parchment. Pound the butter a few times with a rolling pin, then roll from the center outward until the butter is pliable and evenly fills the 9x12-inch space. Chill the butter block for at least 2-3 hours.

Once the dough has just about doubled in size, dump it out on the counter and roll it into a 12 by 18-inch rectangle. Place the dough on a half sheet baking sheet and cover with plastic wrap and refrigerate overnight.

Remove both the butter block and the dough from the refrigerator. Lightly flour your work surface and take the chilled dough out of the pan. Gently roll it into a 12x18-inch rectangle. Unwrap the butter block and place it in the center of the dough. Fold each side of the dough over the butter so the edges meet in the center, pinching the seams to seal the butter inside.

Place the dough seam-side down and roll it into a 10 by 20-inch rectangle. Trim the edges and fold the dough in thirds to form a 10 by 6-inch rectangle. Wrap the dough in plastic wrap and chill in the refrigerator for about 1 hour.

After the dough has chilled, remove it from the refrigerator and roll it into a 10 by 20-inch rectangle again. Trim the edges and fold in thirds to form another 10 by 6-inch rectangle. Wrap well and chill for 1 hour.

Roll the chilled dough into a 24 x 12-inch rectangle, about ¼–½ inch thick. Cut the dough into long triangles and shape each triangle into a croissant.

Cover the croissants lightly with plastic wrap and let them rise at about 78º-80ºF for 10–12 hours, or overnight, until doubled, puffy, and jiggly.

Preheat the oven to 375ºF (190ºC). Whisk together one egg with a splash of cream and a pinch of salt to make an egg wash. Gently brush the croissants and bake for 30–35 minutes until golden brown and flaky.