Add flour, milk, eggs, sugar and vanilla to a blender and blend until thoroughly mixed and frothy.After blending be sure that the batter has come to room temperature and is not too cold. If your batter is too cold it will not rise up high in the skillet.
Melt 2 Tbsp butter in a cast iron skillet in the oven.
Take skillet out of the oven and pour enough batter into the skillet to cover the bottom. Return pan to the oven and bake for 20 minutes, until the cake is puffed and golden. Resist the urge to open the oven, checking through the oven window if possible. When dutch baby is puffed and golden reduce oven temperature to 300° and bake five minutes longer.
Remove dutch baby from the oven. Remove to a plate or platter and cut or tear into wedges. Turn oven back up to 425°. Melt more butter into the skillet and pour in more batter to bake another Dutch baby while you are busy demolishing the first one. Serve hot topped with syrup, jam, confectioners' sugar or fresh fruit.