In a small bowl, combine the lukewarm milk and active dry yeast. Allow it to sit for about 5-10 minutes until it becomes frothy.
In a large mixing bowl, combine the sugar, salt, and 3 tablespoons of unsalted butter. Pour in the activated yeast mixture. Gradually add the all-purpose flour, mixing until a soft dough forms.
Transfer the dough to a lightly floured surface and knead for about 5-7 minutes until it becomes smooth and elastic. Form it into a ball, place it in a greased bowl, and cover with plastic wrap. Let it rise in a warm place for about 1 hour, or until it has doubled in size.
While the dough is rising, prepare the butter block. Take the 1 cup of unsalted butter (2 sticks) and place it between two sheets of parchment paper. Use a rolling pin to flatten it into a rectangle about 1/2 inch thick. Chill this in the refrigerator while your dough rises.
Once the dough has doubled, punch it down and roll it out on a floured surface into a rectangle that is roughly 10x20 inches. Place the chilled butter block in the center and fold the edges of the dough over the butter, sealing it in.
Roll the dough out again into a long rectangle, then fold it into thirds (like a business letter). Wrap the dough in plastic wrap and refrigerate for 30 minutes. Repeat this rolling and folding process two more times, refrigerating between each fold.
Once you’ve completed the lamination, roll the dough out one final time into a large rectangle. Cut the dough into triangles. Starting from the base of each triangle, roll the dough up towards the tip. Place the rolled croissants on a baking sheet lined with parchment paper.
Cover the shaped croissants with plastic wrap and let them proof for about 1-2 hours, or until they have puffed up and are nearly doubled in size.
Preheat your oven to 425°F (220°C). Whisk the large egg and brush it over the tops of the croissants for a beautiful golden finish. Bake for 15-20 minutes, or until they are golden brown and have risen beautifully.
