Preheat your oven to 425°F (220°C). Place a 9x13-inch baking dish in the oven to heat.
In a large mixing bowl, whisk together the flour, milk, eggs, melted butter, salt, and vanilla extract until smooth and well combined.
Carefully remove the hot baking dish from the oven and pour the batter into the dish.
Return the baking dish to the oven and bake for 20-25 minutes, or until the edges are puffed and golden brown.
Once baked, remove from the oven and let it cool for a minute. Cut into squares and serve immediately.
Dust with powdered sugar and drizzle with fresh lemon juice or maple syrup before serving.