In a large mixing bowl, combine flour, salt, sugar, and yeast. Gradually add warm water and milk, stirring until a shaggy dough forms. Let it rest for 30 minutes.

On a floured surface, knead the dough for about 10 minutes until smooth and elastic.

Shape the dough into a ball, place it in a lightly greased bowl, cover with plastic wrap, and refrigerate for at least 4 hours or overnight.

Take cold butter and pound it between two sheets of parchment paper into a rectangle about 1/2 inch thick. Chill again for about 30 minutes.

Roll the chilled dough into a larger rectangle on a floured surface. Place the butter slab in the center and fold the dough over it like an envelope.

Roll out the dough into a long rectangle, then fold it in thirds like a letter. Repeat this rolling and folding process three times, chilling for 30 minutes between each turn.

Roll out the final dough into a large rectangle and cut it into triangles. Starting from the base, roll them up tightly to form croissants.

Place the rolled croissants on baking sheets lined with parchment paper, cover loosely with plastic wrap, and let them rise in a warm spot until doubled in size (about 1-2 hours).

Preheat your oven to 400°F (200°C). Bake your beautiful croissants for 15-20 minutes or until golden brown and flaky.