In a large mixing bowl, combine the all-purpose flour, sugar, salt, and instant yeast. Make a well in the center and add the warm water and whole milk. Mix until a shaggy dough forms. Add the room temperature unsalted butter and knead the dough for about 5-7 minutes until it’s smooth and elastic.
Shape the dough into a ball, place it in a greased bowl, and cover it with plastic wrap. Let it rise at room temperature for about 1-2 hours or until it has doubled in size.
While the dough is rising, prepare the cold unsalted butter. Place the cold butter between two sheets of parchment paper and pound it with a rolling pin until it forms a rectangle, about 1/2 inch thick. Refrigerate the butter block while the dough rises.
Once the dough has risen, punch it down and roll it out on a floured surface into a rectangle, about 12x24 inches. Place the cold butter block in the center of the dough. Fold the dough over the butter, encasing it completely.
Roll the dough out to about 1/4 inch thick and fold it into thirds like a letter. This is your first fold. Wrap it in plastic wrap and refrigerate for 30 minutes.
Roll out the dough again, perform another letter fold, and refrigerate for 30 minutes. Repeat this process for a total of three folds, ensuring you give the dough time to rest between each fold.
After the final fold and rest, roll the dough out to about 1/4 inch thick. Cut it into triangles, about 5 inches wide at the base. Starting from the base of each triangle, roll it up tightly towards the point. Bend the ends slightly to create the crescent shape.
Place the shaped croissants on a lined baking sheet, leaving space for them to expand. Cover them loosely with plastic wrap and let them rise for another 1-2 hours until they have puffed up.
Preheat your oven to 400°F (200°C). In a small bowl, whisk together the egg and heavy cream. Brush this mixture over the croissants for that golden-brown finish.
Bake the croissants for 15-20 minutes or until they are golden brown and flaky. Remove them from the oven and let them cool slightly before enjoying.