In a large mixing bowl, combine the flour, sugar, yeast, and salt.

Add the cold, sliced butter and incorporate it into the flour mixture using your fingers or a pastry cutter until it resembles coarse crumbs.

Pour in the cold milk and stir until a shaggy dough forms.

Turn the dough onto a floured surface and gently knead it until smooth.

Roll the dough into a rectangle and fold it into thirds, like a letter.

Roll out the dough again and fold it three more times to create layers.

Roll the dough into a thin sheet and cut it into long triangles.

Place your choice of filling at the base of each triangle (if desired) and roll the triangles from base to tip.

Place the rolled croissants on a baking sheet, cover, and let them rise for 30 minutes.

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

Bake for 15-20 minutes or until golden brown and crispy.