Heat oven to 425° with a rack in the lower third.
In a bowl, whisk together milk, eggs and salt. Add flour and whisk until smooth.
Melt butter in a 10- to 12-inch castiron skillet or other ovenproof pan set over medium heat. Swirl the pan so the butter coats the bottom and sides. Pour the batter into the skillet and cook, without stirring, for 1 minute. Place the skillet in the oven and bake for 15 minutes. Resist the urge to open the oven door, for the pancake could deflate. Lower heat to 350° and continue baking until the pancake is puffed and golden brown, 10 to 15 minutes more. Serve immediately, dusted with confectioners’ sugar or other toppings.
TO MAKE APPLE DUTCH BABY:
Melt 4 tablespoons butter in a castiron skillet over medium-high heat. Add 2 halved, cored and sliced apples, 2 tablespoons sugar and ½ teaspoon ground cinnamon. Cook, stirring occasionally, until the apples are tender and golden, about 5 minutes. Remove from the heat and proceed with the batter above, pouring the batter over the apples.