To a stand mixer (or a bowl if you are kneading by hand), mix together the yeast, sugar and warm milk. Let it sit for a minute and use your nose to see if the yeast is working (It will smell very "yeasty" once the sugar and warm milk are mixed in. Technically using instant yeast you can omit this "proofing" step and throw the yeast in the with the flour, but I like to double check that my yeast is working so I do it anyway).
Add the room temperature melted butter. Make sure it is not too hot so it doesn’t kill the yeast.
Add the sourdough discard, salt and cornstarch. Begin mixing with your dough hook as you add in the flour a cup at a time. Reserve the half cup of flour to mix in toward the end as you look for readiness of the dough. If the dough is pulling away from the sides, you may not need to add more flour. If the dough is still very sticky, add a little more flour until it is tacky, pulls away from the sides and rolls up into a ball in your fingers (a little stickiness is okay!). This post may help you to check for readiness of dough.
Knead the dough for about 5 minutes in the stand mixer or 7-10 minutes by hand. I like to set a timer and let the dough hook or mixer do the work for 5 minutes or so.
Add a drop of oil to a bowl. Shape the dough into a ball and place into the bowl. Roll the ball around until it is completely covered in the oil. (The oil keeps the dough moist as it rises and makes it easier to handle once risen). Cover the bowl with plastic wrap or a kitchen towel and let rise until doubled in size. If your sourdough discard is cold, it may take longer for the dough to rise. You may want to turn your oven light on and let the dough rise covered in the oven (just make sure not to turn the actual oven on during this process).
Cover the bottom of two 8 or 9 inch cake pans with parchment paper. Alternatively, prepare a half sheet pan (affiliate link for my favorite pans) with parchment paper.
After the first rise, turn the dough out onto the counter and punch it down. Using a bench scraper or a knife, separate the dough into 16 equally-sized pieces.
Roll each piece into a ball and place eight dough balls into each pan, for a total of 16 rolls. If you are baking on a half sheet pan, place all the rolls on the same pan. Cover and let rise again in a warm place for about an hour until just about doubled in size.
Preheat your oven to 350 degrees. Bake for about 22-25 minutes until lightly browned on top. Cover the tops with melted butter (I take a stick of cold butter and lightly touch it on the tops of all the rolls). Enjoy warm or freeze for later!