Preheat oven to 425 degrees. Cut up butter into small pieces, and place in cast iron Dutch oven or skillet without the lid on. Place Dutch oven in hot oven. Prepare batter in a blender by placing eggs in and blending until foamy. Add milk, flour, salt and vanilla and blend until combined. When butter has fully melted in skillet, carefully remove hot Dutch oven from oven and swirl butter to coat pan. Quickly pour batter in skillet and place back in hot oven. Bake for about 25 minutes, or until puffed and lightly browned. Remove from oven and cut into wedges and place on serving plates. Cut tablespoon pats of butter, and place a pat on each serving. Cut lemon in half, and squeeze juice over wedges and sprinkle liberally with powdered sugar. Serve immediately.
Make sure oven is preheated to correct temperature - Be very careful when taking the Dutch Oven out of the oven, it is very hot and you could burn yourself. When you squeeze lemons over the wedges of Dutch Baby, strain though a sieve to catch any seeds and pulp.