In a small bowl, dissolve yeast in warm water with a pinch of sugar. Let sit for 5-10 minutes until frothy.

In a large bowl, combine flour, remaining sugar, and salt. Add the yeast mixture and milk. Mix until a dough forms. Knead for about 5 minutes until smooth. Cover and refrigerate for 1 hour.

Place the cold butter slices between two sheets of parchment paper. Use a rolling pin to flatten and shape it into a 6-inch square. Refrigerate until firm.

Roll out the chilled dough into a 10-inch square. Place the butter square in the center of the dough, positioning it like a diamond. Fold the corners of the dough over the butter, meeting in the center. Seal the edges.

Roll the dough into a 20x10-inch rectangle. Fold the dough into thirds like a letter. Wrap in plastic wrap and refrigerate for 1 hour. Repeat this rolling and folding process two more times, refrigerating for 1 hour between each fold.

Roll the dough into a 24x8-inch rectangle. Cut the dough into 8 triangles. Roll each triangle from the wide end to the tip, forming a crescent shape. Place on a baking sheet lined with parchment paper.

Cover and let rise at room temperature for 1-2 hours, until doubled in size.

Preheat the oven to 375°F (190°C). Brush the croissants with the beaten egg. Bake for 15-20 minutes, until golden brown and flaky.