To the bowl of a stand mixer (or a large bowl if doing by hand) add the active sourdough starter, softened butter, honey, salt, and water. Mix to combine.
Add in the flour, 1/2 cup at a time. Knead until the dough is smooth and elastic and pulls away from the sides of the bowl, about 5 to 10 minutes.
Place the dough into a non-metal bowl and cover. Allow to rise for at least 6 to 8 hours or even better, 10 to 12 hours, in a warm spot.
Divide the dough into 8 equal portions. You can use a bench scraper to do this or your hands.
Use your hands to roll each piece of dough into a ball. Set the dough balls onto the parchment or baking mat-lined sheet pan.
Press down on each ball of dough with your hand to flatten slightly, into the shape of a hamburger bun. The buns will rise more so don't worry if they look small!
Cover with a tea towel and set aside in a warm place to rise for another 1-2 hours.
When ready to bake, preheat the oven to 375 degrees F. Brush on the optional egg wash.
Bake for about 10-15 minutes or until golden brown.
Cool baked buns on a cooling rack before slicing.