This recipe is fool proof as long as you follow the easy steps closely. Preheat your oven to 425 degrees. I put my popover pan in the oven while it preheats (do not grease the pan while it is preheating in the oven).
While your oven/pan is preheating, whisk together:
1/4 teaspoon salt
1 teaspoon vanilla
1 cup of milk (see note)
1 tablespoon melted butter
NOTE: If I have whipping cream on hand, I put about 1/4 cup of whipping cream in a measuring cup and then fill the rest of the way with 2% milk...if I have no cream on hand, I just use 1 cup of 2% milk).
After the above ingredients are whisked together well, add one cup of all purpose flour (spoon flour into measuring measuring cup and level off with straight edge). Whisk everything together for about 30 seconds or until it looks very smooth and creamy.
Take the popover pan out of the hot oven and spray each cup with vegetable spray. Put a small chunk of chilled butter (roughly 1/2 teaspoon but the exact size is not crucial) into the bottom of each HOT cup. Fill each cup half way full with batter (makes six big popovers).
Bake popovers at 425 for 20 minutes. After 20 minutes, turn the oven down to 325 and bake for another 15 minutes.
Remove popovers from pan and poke a tiny hole (I use a toothpick) in the bottom of each popover to relieve a little steam (not totally necessary if you are going to eat them right away).
NOTE: If you do not have a popover pan, you can use 6 ounce (oven safe) custard cups (I used the clear Pyrex ones, for a long time, with great success). I placed them all on a cookie sheet for easier handling. The popovers will not get quite as tall (as they do with a popover pan) but they taste every bit as good.
Someone from outside the USA wrote and asked me what a popover pan was, so here is a photo. As you can see, it is almost like a cupcake pan, but the cups are a lot deeper and spaced far apart so the hot air can circulate around each cup.