To feed the starter: 

1. Combine starter with water and flour in a jar or bowl
2. Let sit at room temperature covered with a loose lid or cloth until starter is bubbly and doubled in volume (2-4 hours). This is called “active starter”

To make the buns: 

1. Stir flour, sugar, and salt together
2. Add water and active starter, and stir until a dough forms
3. Stir or use hands to knead dough until flour is all incorporated
4. Cover, and let rest for 30 minutes
5. Uncover, and using floured hands, grab one side of the dough and stretch, folding over toward the center
6. Turn bowl 1/4 of a turn, and repeat until you’ve stretched all four sides of the dough
7. Cover, and repeat process twice more for a total of three “stretch and folds”
8. After the third stretch and fold, cover dough, and let rise or bulk ferment until dough has doubled in size – 4-6 hours. At this point, you may also cover it with plastic wrap and refrigerate to halt fermentation for 1-2 days
9. After dough has risen, separate into 12 equal sized balls
10. Place balls on parchment lined baking sheets a few inches apart, and flatten down to about the diameter of a hamburger bun
11. Cover with a towel and let rise until doubled – 2-3 hours
12. Preheat oven to 350º
13. Whisk egg with 1 Tablespoon of water, and brush tops of buns
14. Sprinkle with sesame seeds
15. Bake in center of oven for 25 minutes, until lightly browned
16. Remove from oven and let cool completely
17. Use a bread knife to slice open, and serve