In the bowl of an electric mixer, using the paddle attachment mix sourdough starter, milk, egg, salt, honey and 300grams of the flour on medium speed until a loose saggy dough forms. Cover with plastic wrap for 30 min to rest.
Using the dough hook, knead dough for 10+ minutes while you add the remaining flour, butter. The dough should be soft and sticky but still pull away from the edges as it kneads. Add a little more flour if after 10 min it is still very stuck to the bottom / sides of bowl.
Cover with plastic wrap and place in a warm spot (I love the oven with the light on but no heat!) After about 4 hours (it won't necessarily double in size and that's okay!) divide dough into 8 equal portions (about 100grams each).
Shape dough by pulling the edges of each ball into the center and cupping it in your hand to form a tight ball. Place each on a parchment lined baking sheet. Cover with a towel or plastic wrap and let them rise in a warm place for about 2 hours.
To bake, preheat oven to 375. Mix the egg white with a tablespoon of water to create an egg wash to brush over each bun. Sprinkle with sesame seeds and bake for 20 minutes until golden brown. Cool for at least 15 minutes before slicing