1 large egg
2 tablespoons unsalted butter, melted, plus extra for frying
3/4 cup milk
2 teaspoons vanilla extract
2 tablespoons sugar
1/3 cup all-purpose flour
1/4 teaspoon salt
4 to 5 3/4-inch-thick slices day-old challah bread or 6 to 8
slices day-old sandwich bread
1. Heat 10- to 12-inch skillet (preferably cast-iron) over medium heat for 5 minutes. Meanwhile, beat egg lightly in shallow pan or pie plate; whisk in butter, then milk and vanilla, then sugar, flour, and salt, continuing to whisk until smooth. Soak bread without oversaturating, about 40 seconds per side for challah or 30 seconds per side for sandwich bread. Pick up bread and allow excess batter to drip off; repeat with remaining slices.
2. Swirl 1 tablespoon butter in hot skillet. Transfer prepared bread to skillet; cook until golden brown, about 1 minute 45 seconds on the first side and 1 minute on the second. Serve immediately. Continue, adding 1 tablespoon butter to skillet for each new batch.
Notes: Flipping challah is easiest with tongs, but a spatula works as well. To speed the cooking of large quantities, heat two or more skillets to brown a few batches at once. To vary the flavor of the batter, add 3/4 teaspoon ground cinnamon or 1/2 teaspoon ground nutmeg with the dry ingredients, or substitute almond extract for the vanilla.