Combine bread flour, granulated sugar, yeast, and fine sea salt in the mixing bowl of a 5.5-quart stand mixer. Use a whisk to evenly distribute the ingredients.

Attach the dough hook to the mixer and begin mixing on low speed. Slowly pour in the cool water in a steady stream. Allow the mixture to come together into a dough over two minutes. Then, increase the mixer speed to medium and knead for approximately five minutes, or until the dough pulls away cleanly from the bowl’s edges.

Lightly dust a silpat mat or parchment paper with flour. Shape the dough into a rectangle and transfer it to a baking sheet. Cover it fully with plastic wrap to prevent drying. Let the dough chill in the refrigerator for a minimum of four hours, or overnight for better results.

Fold a large sheet of parchment paper into an 8x10-inch rectangle. Alternatively, use two smaller sheets to create the same size. Slice the salted butter lengthwise to make it thinner, then place it inside the folded parchment. Use a rolling pin to flatten and shape the butter into an even layer matching the rectangle dimensions.

Refrigerate the butter block for 5-10 minutes while preparing the dough. Ensure the butter is pliable firm enough to hold its shape but soft enough to bend without breaking.

Roll out the chilled dough on a lightly floured surface to a rectangle measuring 16x10 inches. Retrieve the butter block and position it at the center of the dough so its 10-inch side aligns with the dough’s width.

Fold the dough’s top third over the butter, then fold the bottom third up to cover it, creating a sealed layer. Pinch the edges to secure the butter inside.

Rotate the dough 90 degrees and gently roll it out again to a 16x10-inch rectangle. Fold the dough like a letter, bringing the top third down and the bottom third over it. Wrap the folded dough completely in plastic wrap and refrigerate for 10 minutes.

Repeat this folding and rolling process two more times, chilling the dough for 10 minutes between each step. After the final fold, wrap the dough tightly and refrigerate it for at least two hours, preferably four or more, to allow the layers to rest.

Take the rested dough out of the refrigerator and let it sit at room temperature for about 15 minutes to soften slightly. Roll it out to a large rectangle approximately 24x12 inches. Trim the edges to create clean, straight lines.

Cut the dough into long, slender triangles using a knife or pizza cutter, ensuring each triangle spans the width of the rectangle. Aim for 12 larger croissants or 16 smaller ones.

Flatten the narrow tip of each triangle slightly, then roll it tightly starting from the wide base. Place each croissant with its tip tucked underneath to seal.

Arrange 4-6 croissants on a parchment-lined baking sheet, leaving adequate space for expansion. Loosely cover the tray with plastic wrap, ensuring it doesn’t press against the croissants. Let the croissants proof at room temperature (around 70°F) for roughly two hours.

The croissants are ready for baking when they have increased in size and a gentle press leaves a slight indent. They should wobble slightly when the tray is gently shaken.

Preheat the oven to 400°F (204°C) for convection baking, or 375°F (181°C) for a conventional oven. Beat the egg in a small bowl, then brush each croissant generously with the egg wash. Apply a second coat for a glossy finish.

Bake the croissants for 15-18 minutes, or until they turn a deep golden brown. Properly baked croissants should feel light and airy when picked up. Allow them to cool on the tray for 10-15 minutes before transferring to a wire rack to cool completely.

Serve croissants at room temperature on the day of baking for the best flavor and texture. Store leftovers in an airtight container for 2-3 days at room temperature, or refrigerate for up to a week.