Step 1: Prepare the Dough

1. Combine flour, salt, and sugar in a large bowl.
2. Dissolve the yeast in lukewarm milk and let it sit for 5 minutes until frothy.
3. Add the yeast mixture to the flour mixture along with cold water, and mix to form a dough.
4. Knead the dough for 8-10 minutes on a floured surface until smooth and elastic.
5. Shape the dough into a ball, cover, and let it rise for 1-1.5 hours until doubled in size.

Step 2: Prepare the Butter Block

1. Place cold butter between two sheets of parchment paper and roll it into a 7-inch square using a rolling pin.
2. Refrigerate the butter block while the dough rises.

Step 3: Laminate the Dough

1. Punch down the risen dough and roll it into a 10-inch square.
2. Place the cold butter block in the center and fold the dough over the butter to seal it inside.
3. Roll the dough into a 24×10-inch rectangle.
4. Fold the dough into thirds (like a letter), wrap it in plastic wrap, and chill for 30 minutes.
5. Repeat the rolling and folding process two more times, chilling for 30 minutes between each fold.

Step 4: Shape the Croissants

1. Roll the dough into a large rectangle, about 1/4 inch thick.
2. Cut the dough into triangles (4 inches wide, 8 inches tall).
3. Roll each triangle tightly from the base to the tip to form the croissant shape.
4. Place the croissants on a parchment-lined baking sheet, tucking the tips under to keep them from unrolling.

Step 5: Proof the Croissants

1. Cover the croissants with a towel and let them rise for 2 hours until doubled in size.

Step 6: Bake the Croissants

1. Preheat the oven to 400°F (200°C).
2. Brush the croissants with the egg wash.
3. Bake for 20-25 minutes until golden brown and flaky.
4. Cool slightly before serving.