This brioche is made with sourdough starter for healthy fermentation and a bit of tangy flavor!
In the bowl of a stand mixer, place the milk, sourdough starter, eggs, flour, and sugar. Knead on medium-low using the dough hook for 5 minutes.
With the mixer still running, add the salt.
Add the butter, one tablespoon at a time, waiting until the previous tablespoon has been kneaded in before adding the next one.
Continue to knead the dough for an additional 15-20 minutes, or until it passes the windowpane test. Grab a piece of the dough and stretch it out. If you can see light through it before it tears, it passes!
Scrape the dough into a greased glass bowl. Cover and allow to ferment at room temperature for 1 hour. Place in the refrigerator and allow to chill for 12 hours.
Grease two 9x5-inch glass loaf pans. Set aside.
Divide the dough in 2 and form each half into a loaf. You can braid the loaves or form them into 6 individual buns. I personally recommend braiding them.
Tuck the loaves into the prepared loaf pans. Cover and allow to rise until they puff up over the edges of the pans, about 2-8 hours.
30 minutes before the loaves are done rising, preheat your oven to 375° F (190° C).
If you'd like to do an egg wash, whisk together the egg and water. Brush on top of the risen loaves.
Bake in preheated oven until the loaves are deeply golden and an instant read thermometer inserted in the center registers 190-195°F (88-90°C), about 35-40 minutes.
Remove loaves to a wire cooling rack. Cool for at least an hour before slicing.