1. In a bowl, mix warm milk, sugar, and yeast. Let it sit for 10 minutes until frothy.
2. In another bowl, combine flour and salt. Add the yeast mixture and mix until a dough forms.
3. Knead the dough for about 10 minutes until smooth. Cover and let it rise for 1 hour.
4. Roll out the dough into a rectangle. Place butter pieces on one half of the dough, then fold the other half over the butter. Seal the edges.
5. Roll out the dough again into a rectangle, then fold it into thirds. Repeat this process 3 times, chilling the dough for 30 minutes between each fold.
6. Roll out the dough one last time and cut it into triangles. Roll each triangle from the base to the tip to form croissants.
7. Place the croissants on a baking sheet lined with parchment paper. Brush with beaten egg.
8. Bake at 400°F (200°C) for 15-20 minutes until golden brown.