In a large mixing bowl (or stand mixer bowl), combine flour, sugar, salt, and yeast. Whisk to combine. Add lukewarm milk and cold water. If using a stand mixer, mix with a dough hook on low speed for 2-3 minutes until a shaggy dough forms. If mixing by hand, stir with a wooden spoon.

Add the softened butter and continue mixing on low speed (or kneading by hand) for 8-10 minutes, until the dough is smooth, elastic, and pulls away from the sides of the bowl. It should be slightly sticky.

Transfer the dough to a lightly oiled bowl, cover tightly with plastic wrap, and let it rise in a warm place for 1.5-2 hours, or until doubled in size.

Punch down the dough gently, reform into a ball, and wrap tightly in plastic wrap. Refrigerate for at least 4 hours, or preferably overnight (up to 12-18 hours). This cold fermentation helps with flavor and makes the dough easier to handle for lamination.

While the dough is chilling, prepare the butter block. Cut the very cold 250g unsalted butter into 1/2-inch thick slices. Arrange them closely together on a piece of parchment paper to form an approximately 6x6 inch (15x15 cm) square.

Cover with another piece of parchment paper and use a rolling pin to gently pound and roll the butter until it forms an even 8x8 inch (20x20 cm) square, about 1/4 inch (6mm) thick. Ensure there are no gaps. Place the butter block, still wrapped in parchment, in the refrigerator for at least 30 minutes, or until ready to use.

Lightly flour your work surface. Roll the chilled dough into a 16x8 inch (40x20 cm) rectangle. The dough should be twice as long as the butter block is wide. Place the chilled butter block in the center of the dough. Fold the exposed dough over the butter, completely encasing it. Pinch the seams closed to seal the butter inside.

Gently roll the dough into a long rectangle, approximately 24x8 inches (60x20 cm). Be firm but careful not to tear the dough and expose the butter. Dust with flour as needed to prevent sticking, but not too much.

Perform the first 'single fold': Fold one-third of the dough over the middle section, then fold the remaining one-third over that, like folding a letter. This creates three layers of dough and two layers of butter. Rotate the dough 90 degrees (so the open seam is on your right).

Wrap the folded dough in plastic wrap and refrigerate for 30 minutes.

Repeat the rolling and folding process ('single fold') two more times, chilling the dough for 30 minutes between each fold. Each time, rotate the dough 90 degrees before rolling to ensure even lamination. After the third fold, wrap tightly and refrigerate for at least 1 hour, or up to 2 hours, before shaping.

On a lightly floured surface, roll the chilled dough into a large rectangle, approximately 16x20 inches (40x50 cm) and about 1/8 inch (3mm) thick. Trim the edges to create clean lines.

Using a ruler and a sharp knife or pizza cutter, cut the dough in half lengthwise (to make two 8x20 inch strips). Then, cut each strip into 6 rectangles, each measuring approximately 3.3x8 inches (8.5x20 cm). You should have 12 rectangles in total.

Place two chocolate batons (or an equivalent amount of chopped chocolate) along one short edge of a dough rectangle. Roll the dough up tightly towards the opposite short edge, ensuring the chocolate is fully enclosed. Place another chocolate baton near the end of the roll and continue rolling to fully enclose it, finishing with the seam on the bottom.

Repeat with the remaining dough rectangles and chocolate. Place the shaped croissants on baking sheets lined with parchment paper, leaving plenty of space between them as they will expand significantly.

For the egg wash, whisk together the large egg and 1 tbsp whole milk.

Loosely cover the baking sheets with plastic wrap or a large, inverted container to create a humid environment. Let the croissants proof in a warm, draft-free place for 2-3 hours, or until they have nearly doubled in size, are visibly puffy, and jiggle slightly when gently shaken. This is a crucial step for flaky croissants.

Preheat your oven to 400°F (200°C) with a rack in the middle position. Once proofed, gently brush the tops of the croissants with the egg wash.

Bake for 10 minutes at 400°F (200°C), then reduce the oven temperature to 375°F (190°C) and continue baking for another 8-10 minutes, or until the croissants are deep golden brown and the layers are clearly visible.

Transfer the baked croissants to a wire rack to cool slightly. Serve warm and enjoy!