Servings: Makes 1 very large loaf, or 2 medium sized loaves.

6 eggs (at room temperature) 6 eggs (at room temperature)
1 cup unsalted butter (2 sticks/½ pound/230g, at room temperature) 1 cup unsalted butter (2 sticks/½ pound/230g, at room temperature)
4 ¾ cups all-purpose flour (620g, divided) 4 ¾ cups all-purpose flour (620g, divided)
1 tablespoon active dry yeast (9g) 1 tablespoon active dry yeast (9g)
¼ cup granulated sugar (50g) ¼ cup granulated sugar (50g)
1½ teaspoons salt (8g) 1½ teaspoons salt (8g)
½ cup warm water (120 ml, about 120 degrees F/49 degrees C) ½ cup warm water (120 ml, about 120 degrees F/49 degrees C)
Nutella Nutella
Egg wash, optional (1 egg + 1 tbsp water, mixed) Egg wash, optional (1 egg + 1 tbsp water, mixed)

Day 1: subheading: Day 1:
Take out 6 eggs and 1 cup / 230g butter a few hours before you begin to bring them to room temperature. Prepare all other ingredients before starting.
In the bowl of an electric mixer fitted with a paddle attachment, mix together 1 cup flour (130g), 1 tablespoon active dry yeast (9g), ¼ cup granulated sugar (50g), and 1½ teaspoons salt (8g), before adding ½ cup (120 ml) warm water (at about 120 degrees F/49 degrees C). Turn the mixer on at medium speed for a couple of minutes until well-combined.
Now add in 1 egg at a time. Only add the next egg when the previous egg is well incorporated.
Set the mixer to low. Add in 2 cups of flour (260g), a ¼ cup at a time, and let the mixer go until the dough is well combined.
Turn the mixer off, and cut the 2 sticks of softened butter into small cubes/chunks. With the mixer on medium, add the butter in 6 separate batches, waiting until the butter is well incorporated after each batch before adding more.
Set mixer to low. Now add in 1 ¾ cups of flour (230 grams), a ¼ cup at a time until the dough is well-combined. This dough will be very sticky (closer to a very thick batter) when it’s done.
Use a rubber spatula to clean the dough off the mixing paddle, and scrape the sides of the mixing bowl. Cover the dough with a plate or plastic wrap and let it proof at room temperature (about 70 degrees F/21 degrees C) for 3 hours, until it doubles in size.
After 3 hours have elapsed, punch and deflate the dough completely with a rubber spatula. Cover the dough with a plate again, and refrigerate it overnight until 3 hours before you’re ready to bake the next day.
Day 2: subheading: Day 2:
Remove the dough from the refrigerator. Since I decided to make a braided knot, I divided the dough into 8 equal pieces for 2 separate loaves.
Shape and roll them on a lightly floured clean surface to form a long, thin rectangle. Spread Nutella across the rolled out dough and roll the dough back up along the long side. Repeat this for all 7 remaining pieces.
Cut all the dough in half (along the length). I refrigerate the dough for a bit before to make it easier to cut. You will have 16 pieces now.
Braid as you desire. I'm not sure if I'm allowed to link to videos but you can go through my comment history to see a link I posted in another comment.
I let my dough rise until puffy (roughly an hour).
Top with egg wash before baking and bake at 350F for 35 to 40 minutes.