Preheat oven to 400°F. Grease a 6-cup popover pan or standard muffin tin with cooking spray.
In a blender, combine the eggs, milk, butter, and salt in a blender until totally smooth. Add the flour and pulse several times, until just combined and just a few small lumps remain. Scrape down the sides of the jar if necessary.
Pour the batter into the prepared pan, filling each cup about 2/3 way full.
Bake for 25 minutes, then reduce the oven to 350°F for 10-15 more minutes, until the popovers are golden brown and crisp on the exterior and puffy.
Remove from the oven and use a sharp knife to poke a small hole in each popover to let steam release. Cool in the pan for 5 minutes, then serve immediately.
Store in an airtight container in the fridge for up to 3 days, or in the freezer for up to 2 months. Reheat in the oven or the toaster oven.