In a large mixing bowl with dough hook attachment, combine all the ingredient for the dough and mix until the dough comes together for about 5 minutes. Transfer the dough onto floured surface and knead for a minute. Flatten it into a disk and place it in the floured dish. Cover with plastic wrap and refrigerate for at least 6 hours, or overnight.
Half the butter sticks lengthwise and put them on a large parchment paper. Place another large parchment paper over the butter and roll into 7x7in square. Trim the edges and place them over the butter and roll again until you get nice and even flat square of butter. Refrigerate for at least 30 minutes, or overnight. Do NOT freeze.
Roll the square dough on the floured surface into a 10x10in square, it doesn’t have to be exact, but close enough. Place the 7x7in butter square on the dough and fold the edges like an envelop, as shown above. Pinch the seams securely.
Roll the dough into 8x24in rectangle. Again, it doesn’t have to be exact measurements, but do your best to be close to that. Fold into thirds and cover with plastic wrap. Refrigerate for 30 minutes.
Repeat the step 4 two more times. If the dough resist too much at any point, refrigerate for 10-15minutes.
If you have long enough countertop surface, roll the dough into 8x44in rectangle on floured surface. Because my countertop is not that big, I cut the dough into 2 equal parts, and rolled each part into 8x22in rectangle. Make sure to lift the dough before final measurement, because it shrinks back a little.
Mark every 5.5in on both sides of the rolled dough. Using a pizza cutter, cut the dough into smaller rectangles, and then cut each rectangle into triangles. You should get 16 triangles total.
Since the triangles are not perfect, slightly stretch one corner to make it more perfect triangle. Slightly roll each triangle if needed. Slit the bottom side of the triangle and roll into croissant shape, as shown about. If desired, you can smear some Nutella before rolling.
Place the croissants on the baking sheets, lined with parchment paper or silicone mat. Let them proof for about an hour.
Preheat the oven to 425°F (220°C).
In a small bowl, beat an egg with a tablespoon of water. Brush the egg wash over the croissants. Bake for 20-25 minutes, until golden brown. Serve warm.