Preheat oven to 425°F with a 10-inch cast-iron skillet inside.
Whisk eggs, milk, sugar, vanilla, and salt until frothy and smooth.
Sprinkle in flour and whisk just until no lumps remain; rest batter 5 minutes.
Add butter to hot skillet; swirl to coat and return to oven until fully melted.
Pour batter into sizzling butter. Bake until puffed and deep golden, 16–18 minutes.
Dust with powdered sugar, squeeze lemon over top, and add berries. Serve right away.