In a large bowl, combine flour, warm water, milk, sugar, yeast, and salt. Mix until a dough forms.

Knead the dough on a lightly floured surface for about 10 minutes until smooth.

Shape the dough into a ball, place it in a lightly oiled bowl, cover with plastic wrap, and let it rise in a warm place for 1–2 hours, or until doubled in size.

Once risen, punch down the dough and roll it into a rectangle about 1/2 inch thick.

Place the cold butter slices evenly over two-thirds of the dough. Fold the unbuttered third over the buttered section, then fold the remaining third over that, like a letter.

Roll the dough into a rectangle again, then fold it into thirds. Wrap in plastic wrap and refrigerate for 30 minutes.

Repeat the rolling and folding process two more times, chilling the dough for 30 minutes between each turn.

After the final fold, roll the dough into a 1/4-inch thick rectangle. Cut the dough into triangles.

Starting from the wide end, roll each triangle tightly into a crescent shape. Place the croissants on a baking sheet lined with parchment paper.

Cover and let them rise for 1–2 hours, or until doubled in size.

Preheat the oven to 400°F (200°C). Brush the croissants with the beaten egg.

Bake for 15–20 minutes, or until golden brown.

Cool on a wire rack before serving.