Preheat your oven to 425°F (218°C).
Place the butter in a 9x13-inch baking dish and put it in the oven until melted.
Meanwhile, in a blender or large bowl, mix eggs, flour, milk, salt, and vanilla until smooth.
Carefully remove the hot dish from the oven and pour the batter over the melted butter.
Return the dish to the oven and bake for 20–25 minutes until the pancake is puffed and golden brown.
Serve immediately with powdered sugar, fresh berries, or syrup.