Preheat oven to 45 15 minutes before baking, with the rack in the center of the oven. Have a 1-inch glass pie plate or cast iron skillet ready.
Mix the milk, flour, eggs, sugar and vanilla until smooth, in a food processor, blender or with a whisk. The batter can be made a day ahead and refrigerated. Put the butter in the pie plate or skillet and place in the hot oven until the butter is melted. Brush the butter up the sides and onto the rim of the pan so the entire inside surface is well coated. Slowly pour the batter into the pie plate or skillet and bake for 2 minutes. Reduce the oven to 35 F and continue baking until the pancake is well browned and cooked in the center, 8 to 1 minutes longer.
Remove from the oven and sift confectioners sugar over the top. Serve immediately, with fruit spooned into the center or with lemon wedges