In a small bowl, dissolve the yeast in warm milk and let it sit for about 5-10 minutes until it becomes frothy. This step ensures that the yeast is active and ready to help your dough rise.

In a large mixing bowl, combine the flour, sugar, and salt. Add the softened butter and mix with your fingers until the mixture resembles coarse crumbs. Pour in the yeast mixture and knead until a smooth dough forms. This can take about 8-10 minutes by hand or 5 minutes in a stand mixer. Wrap the dough in plastic wrap and refrigerate for at least 1 hour.

Place the cold butter between two sheets of parchment paper and roll it into a 7x7-inch square. Refrigerate until firm but pliable, about 30 minutes.

On a floured surface, roll out the chilled dough into a 10x10-inch square. Place the butter block in the center and fold the corners of the dough over the butter, enclosing it completely. Roll the dough into a 10x20-inch rectangle, then fold it into thirds like a letter. Wrap it in plastic wrap and chill for 30 minutes. Repeat this process two more times to create multiple layers.

Roll out the dough to a 10x20-inch rectangle and cut it into triangles (about 4 inches wide at the base and 8 inches tall). Gently stretch each triangle and roll it from the base to the tip, forming a crescent shape. Place the croissants on a baking sheet lined with parchment paper, ensuring enough space between them for rising.

Cover the croissants with a kitchen towel and let them rise at room temperature for about 2 hours or until doubled in size. Avoid placing them in a warm oven, as too much heat can cause the butter to melt, reducing the flakiness.

Preheat your oven to 375°F (190°C). Whisk together the egg and milk to create an egg wash. Lightly brush the croissants with the mixture, giving them a golden finish when baked.

Bake the croissants for 18-22 minutes or until golden brown and crispy. Let them cool on a wire rack before serving.