Lightly flour a cool, clean surface. Roll the dough out into a rectangle that is roughly 9 x 23". You can trim the sides to get an exact measurement if desired. Use a ruler to measure 1 and ½" in from the top left. Make a small cut with a pastry wheel to mark the spot. From there, make a small cut every 3". On the bottom starting from the left, make a small cut every 3".
Make a cut diagonally from the bottom left corner to the first 1.5" cut. This is scraps and can be thrown away. From there, form the croissant triangles by making diagonal cuts connecting each mark.
Arrange the croissants on a baking pan lined with parchment paper or a nonstick baking mat. Leave a little room between them because they will expand during baking. You may need a second pan if they don't all fit on one.
Allow the croissants to rest for 1 hour in the fridge before baking them.
Preheat oven to 400°.
Make the egg wash: Mix 1 egg + 1 tablespoon of milk in a small bowl. Brush each of the croissants with the egg wash before baking.
Bake the croissants in the preheated oven for 15-20 minutes. Halfway through baking rotate the pan, which helps make sure they brown evenly. Remove the croissants from the oven and serve warm. Croissants are best fresh out of the oven, enjoy!