1. In a bowl, mix warm milk, sugar, and yeast. Let it sit for 10 minutes.
2. Add flour and salt to the yeast mixture. Knead until smooth.
3. Roll out the dough, place cold butter in the center, and fold the dough over it.
4. Roll out the dough again, fold it into thirds, and chill for 30 minutes. Repeat this process 3 times.
5. Roll out the dough, cut into triangles, and roll each triangle into a croissant shape.
6. Place croissants on a baking sheet lined with parchment paper. Let them rise for 1 hour.
7. Preheat the oven to 400°F (200°C). Brush croissants with beaten egg.
8. Bake for 15-20 minutes until golden brown.