In a medium bowl (with pour spout, if possible), lightly whisk eggs. Add milk, sourdough starter, and salt. Whisk lightly until just combined.
3 Gently mix in flour until large lumps disappear, being careful not to overmix. A few small lumps are okay. Batter will be very thin, about the consistency of heavy cream.
4 Once popover pan has been heated, carefully remove it from the oven and spray with cooking spray.
5 Pour or ladle batter evenly between cups. For a popover pan, fill each cup about halfway. For a muffin pan, fill each cup almost to the top. Should make 6 popovers or 8 muffins cups.
6 Place pan in oven and bake for 15 minutes at 450 degrees.
7 Without removing popovers from oven, reduce temperature to 375 degrees and bake an additional 15-20 minutes. Bake as long as possible without letting them get too dark.
8 Remove popovers from oven and pierce each with a sharp knife tip to release steam.
9 Serve immediately with whipped butter.