In a mixing bowl mix together yeast, water, and sugar. Let sit for 5-7 minutes, until it begins to get frothy.
Whisk in egg and butter.
Add flour and salt and mix with a dough hook on medium-low speed for 6-7 minutes, until smooth and elastic.
Oil a large bowl and transfer the dough into it. Spray plastic wrap with cooking spray and cover the bowl with the sprayed side facing the dough. Let rise until doubled in size, about 40-60 minutes.
Preheat oven to 425.
When dough has doubled in size, cut into 8-10 equal portions (you may use a kitchen scale for this, but I just eyeballed it). Form your dough into balls, keeping the seam at the bottom of the rolls. For flatter buns, use your hands to flatten and stretch into more of a disk shape than a round ball of dough. Place on parchment lined sheet pans, leaving 2 inches between each bun.
Let rest for 10 minutes.
Mix together egg yolk and water for the egg wash. Use a pastry brush to fully brush the top of each bun. Allow the buns to sit for a minute or two and then brush again with the egg wash to get darker color on them. Be sure the brush down the sides of them, too. Sprinkle with sesame seeds, if desired.
Bake for 10 minutes, rotate the pan, and then bake for 2 more minutes. Allow to cool completely before serving.