In a bowl, mix the flour, yeast, sugar, and salt. Heat the milk until warm but not boiling, then pour it into the dry ingredients. Mix until a dough forms.

Knead the dough gently for about 5-7 minutes until smooth. Place it in a greased bowl, cover, and let it rise in a warm place for about 1 hour, or until doubled in size.

While the dough is rising, take the cold butter and place it between two sheets of parchment paper. Use a rolling pin to flatten it into a rectangle about 1/2 inch thick. Refrigerate it until the dough has risen.

Once the dough has risen, roll it out on a floured surface into a large rectangle. Place the butter block in the center. Fold the dough over the butter to encase it completely.

With the butter sealed inside, roll the dough out into a long rectangle, then fold it into thirds, like a letter. Repeat this rolling and folding process two more times, refrigerating in between if the dough becomes too warm.

After your final fold, roll the dough out once more into a large rectangle. Cut it into triangles. Starting from the base of each triangle, gently roll it up toward the tip.

Place the rolled croissants on a baking sheet lined with parchment paper. Let them rise for about 30-45 minutes. Brush with beaten egg for a beautiful finish. Bake in a preheated oven at 400°F (200°C) for about 15-20 minutes or until golden brown.