Day 1 (Morning): Feed your sourdough

1. Feed your sourdough starter and let it rest 4-6 hours, or longer, until fully active

Day 1 (Evening): Prepare the dough

1. Whisk the flour, brown sugar, and salt in a large bowl. Use your fingertips to rub the butter into the flour mixture, breaking it up into tiny flour-coated pieces the size of breadcrumbs. Stir in the sourdough culture and milk using a wooden spoon or rubber spatula to form a shaggy mass.
2. Turn the dough out onto an unfloured work surface and knead eight to ten times, until all of the flour is just incorporated. You don’t want to over work it, because you don’t want the butter to melt too much. The dough will not be a smooth mass; you will see some flecks of butter. It should be soft and tacky, but not sticky. Adjust as needed with flour or water.
3. Place the dough in a large, clean bowl. Cover with plastic wrap. Place in refrigerator for 8 to 24 hours (24 will give you the best flavor).

Day 2 (Morning): Make the butter block

1. Knead the butter and flour together using your hands until it forms a smooth mass. This will take a couple of minutes. The butter should be pliable. You can use a stand mixer, but make sure you don’t beat any air into the butter or melt it.
2. Spread the butter between 2 large sheets of plastic wrap (or parchment or wax paper), and use a rolling pin to shape into a rectangle that is about 8″x9″. Use a straight edge to form corners, but work quickly as you want the butter to stay cool. Wrap in plastic and refrigerate until you’re ready to roll out the dough.

Day 2 (Afternoon):

First turn:

1. Scatter a little bit of flour on your work surface, then turn the dough out onto it. Roll it out into a rectangle that is 10″ by 15″ and about 1/4″ thick. Using your hands, gently pull and stretch the dough to form straight edges and sharp corners. Brush excess flour off of the dough. Set the dough with a long edge facing you.
2. Mentally divide the dough into 3 equal portions. Place the butter block over the right 2/3 of the dough, leaving a 1″ border on the outer edges. Fold the empty left portion of the dough over the middle third. Now, lift and fold the right section of dough over that. You should have 3 layers of dough that encase 2 layers of butter. Pinch the outsides and the seams together and lightly press the layers together using a rolling pin. This completes the first turn. Wrap in plastic and refrigerate for 1 hour.

Second turn:

1. Remove the dough from the fridge and set it on your lightly floured work surface. Roll dough out into a 10″ by 20″ rectangle, pulling and stretching to form straight edges and sharp corners. Brush off any excess flour. Set the dough with a long edge facing you. Fold both of the short ends in to the center, leaving a 1/4″ gap where they meet (think of a book jacket). Fold one side of the dough over the other. Lightly press the layers together using a rolling pin, and square and sharpen the edges and corners. This completes the second turn. Wrap in plastic and refrigerate for 1 hour.

Third (and final) turn:

1. Lightly dust your work surface and the top of the dough with flour. Roll dough out into a 10″ by 15″ rectangle. Do another trifold, as done in the first turn (mentally divide into thirds, then fold one third over the center, followed by the last third). Square the edges and sharpen the sides; wipe off excess flour. Wrap in plastic and refrigerate for at least 2 hours, but up to another 24 hours.

Make ahead and freeze dough for later:

1. If you want to save the dough for later, wrap it tightly in plastic wrap, slide it into a freezer baggie, and freeze for up to 1 week. Defrost overnight in the refrigerator before proceeding to final shaping.

Day 3 (Afternoon) Final shaping:

1. Line two rimmed baking sheets with parchment paper and set aside. (You will need rimmed baking sheets to capture the melted butter.)
2. Lightly dust your work surface and top of your dough with flour. Roll dough out into a 15″ by 18″ rectangle (~1/4″ thick). Pull and stretch to form straight edges and sharp corners. Patch any holes where butter may have popped through by dusting them with flour. Brush any excess flour off the dough.
3. Cut the rectangle in half widthwise, creating two 15″ x 9″ sheets of dough. Using a pizza cutter or bench scraper, cut each piece of dough into three equal strips, the short way.
4. Cut each strip in half diagonally, so you have 6 triangles. Repeat with other piece of dough for a total of 12 triangles.
5. Beginning at the base, roll the triangles up, tugging on the tip to elongate it slightly, then gently pressing it into the dough.
6. Place on the prepared baking sheets with the tips tucked under, and curve the ends to form crescent shapes. Cover the croissants with damp, clean kitchen towels and allow to rise at cool room temperature until they have almost doubled in size and feel spongy, ~2 hours.
7. Slide the croissants into the refrigerator for 20 minutes while you prepare the dipping solution.
8. Preheat oven to 425° F, positioning one rack in the upper third of the oven, and one in the lower third.

Prepare the dipping solution:

1. Add the baked baking soda in 8 cups of cold water and stir until completely dissolved. One by one, dip the croissant dough into the dipping solution, allow the excess to drip off, then set back on the lined trays.

Finish them off:

1. Brush the tops with the egg wash, then sprinkle with coarse salt and sesame seeds or poppy seeds, if using.
2. Slide into preheated oven immediately and bake for 14-18 minutes (rotating pans from front to back and top to bottom halfway through), until they are deeply browned, crispy, and flaky. They should feel light and airy if you pick them up.
3. Transfer to a wire rack to cool for 10 minutes before serving. They are best enjoyed the day they are made, ideally warm from the oven. Store any extras in a paper bag for a day. You can reheat them by placing them in a 350° F oven for ~5 minutes.