Combine the flour, water and sourdough starter in stand mixer. Mix well. It will be sticky, sticky is okay, but it should be able to form a ball.
After the dough has rested, for 30 minutes, add the ingredients for the yeast mixture in a separate bowl and let it activate for about 3-5 minutes. (Acitivated yeast will look bubbly) Then put the activated yeast, and the salt in the stand mixer with the dough and mix for 5 minutes on low, or knead the ingredients together for about 10 minutes. The dough should be sticky but still in a ball. You may need to add 1/2 cup more flour if your dough is really sticky!
After it had been mixed, it should have more of a smooth texture. Now, let it rise for 1 hour- 1 1/2 hours.
After it has risen, cut the dough into 8-12 pieces, depending on how big you like your rolls, and place on a baking sheet.Score the top if you would like and cover those and let it rise one more time for another 30 minutes.
Right before they go in the oven, brush over the rolls with the egg yolk
Bake at 450 for 15-17 minutes