1.Heat the oven to 375 degrees F.
2.Put 1 tablespoon of the butter in a 10-inch cast iron pan and heat the pan in the oven for 10 minutes. Melt the remaining 2 tablespoons of butter and set aside to cool slightly.
3.Pulse together the flour, sugar and salt in a blender. Add the eggs, milk, vanilla extract and 2 tablespoons melted butter, and blend the batter until smooth and frothy, 30 to 45 seconds.
4.Carefully remove the hot skillet from the oven and immediately pour the batter into the center. Bake for 20 minutes, do not open the oven while baking. The Dutch baby will puff up in the center and the edges will be dark and crispy.
5.Serve warm with a sprinkling of powdered sugar and lemon wedge for squeezing.