Place a cold popover or muffin pan in the oven and heat to 450°F as you prepare the popover batter.
Heat up the milk in a microwave (or in a saucepan on the stove) just until it begins to warm up. You don't want hot, just warm.
In a large bowl, mix together the room temperature eggs, sourdough starter, salt, and warm milk.
Add in the flour, mixing gently with a whisk (making sure to not overmix). The batter should not be very thick (thinner than pancake better) and a few lumps are okay.
Once the popover or muffin pan has been heated, CAREFULLY remove it from the oven and spray it with nonstick spray.
Pour the batter into the cups of the pan. Fill each popover cup almost to the top. (This applies to a muffin pan, if you are using a popover pan fill about half-way.)
Bake the popovers for 15 minutes at 450°F. THEN, reduce the heat to 375°F, baking them an additional 15-20 minutes, or until golden brown. Serve warm with the cinnamon sugar butter.