(from Baking Illustrated)
2 large eggs
1 cup whole milk
1 cup (5 ounces) unbleached all-purpose flour
½ teaspoon salt
1 tablespoon unsalted butter, melted
1 tablespoon vegetable oil
1. In a large bowl, whisk the eggs and milk until well combined, about 20 seconds. Whisk the flour and salt in a medium bowl and add to the egg mixture; stir with a wooden spoon or spatula just until the flour is incorporated; the mixture will still be lumpy. Add the melted butter. Whisk until the batter is bubbly and smooth, about 30 seconds. Let the batter rest at room temperature for 30 minutes.
2. While the batter is resting, measure ½ teaspoon vegetable oil into each cup of the popover pan. Adjust the oven rack to the lowest position, place the popover pan in the oven, and heat to 450°F. After the batter has rested, pour it into a 4-cup liquid measuring cup or another container with a spout (you will have about 2 cups batter). Working quickly, remove the pan from the oven and distribute the batter evenly among the 6 cups in the pan. Return the pan to the oven and bake for 20 minutes, without opening the oven door. Lower the heat to 350°F and bake until golden brown all over, 15-18 minutes more. Invert the pan onto a wire rack to remove the popovers and cool for 2 to 3 minutes. Serve immediately.