In a stand mixer with a dough hook, add sourdough starter, warm water, all purpose flour, melted butter or coconut oil, honey, salt and egg. Knead for 5-10 minutes with dough hook until smooth and elastic.
Place dough in a greased bowl and cover with a damp towel or plastic wrap for 8 hours.
Transfer the dough to the refrigerator and chill covered overnight.
Divide the dough into eight equal pieces.
Shape the buns by creating a flat surface on top and tucking the remaining dough underneath. I also like to create a little tension by rotating each bun slightly on a lightly floured surface.
Cover the buns with plastic wrap or a damp tea towel and allow to rise until doubled.
Beat the egg yolk and water and brush the tops of the buns with the mixture.
Sprinkle with sesame seeds. (optional)
Bake the buns for 25-30 minutes at 350 degrees. You want them to be golden on top.
Notes
*If you plan to make this recipe without long fermenting you can add all the ingredients (including the egg) in a stand mixer, and skip the fermentation process.