In the bowl of a stand mixer, place the milk, sourdough starter, eggs, flour, and sugar. Knead on medium-low using the dough hook until combined (about 5 minutes).
Add your salt to the dough and mix.
Add 1 tbs pads of butter at a time. Let the butter become slightly encorporated before adding the next pad of butter.
Once the butter has been added, continue to let the stand mixer knead for about 5-15 minutes until dough passes the windowpane test (see notes below).
Transfer the dough into a lightly greased bowl (not metal), cover and ferment on counter for 1 hour.
Transfer the bowl to the refrigerator and allow to cold ferment for about 12 hours.
Prepare your 9x5- inch loaf pans by lightly greasing with coconut oil and set aside.
This makes 2 loaves so divide them equally (by weight) and shape into two balls of dough.
Grease a countertop as this dough is quite sticky!
Decide how you want to shape the dough. I prefer to braid the dough.
To braid, split the dough balls into three equal pieces (by weight) and roll out into long strips (about 9 inches). Then take the ends of all three pieces and pinch them together. Take the right piece of dough overtop the middle piece then the left overtop the middle and continue to braid until you reach the end. Pinch the end pieces together.
Place the loaves into your prepared pans. Cover and allow to rise until they puff up over the edges of the pans, about 2-8 hours. My kitchen is about 70 degrees and it took the entire 8 hours.
Preheat your oven to 375° F.
Whisk together the egg and water and brush the egg wash overtop the loaves just before placing in the oven.
Bake in preheated oven until the loaves are golden brown on top. About 30-40 minutes.
Remove loaves to a wire cooling rack and allow to cool for at least an hour before slicing.