Heat oven to 425°. Place the butter in the bottom of a 12-inch skillet (preferably cast iron, and not nonstick) and let it melt in the oven. Meanwhile, combine the milk, flour, eggs, sugar, lemon zest, vanilla bean, and salt in a blender. Blend until smooth. When the butter has melted and the pan is warm, gently swirl the pan so the bottom is coated with butter. Pour in batter and bake until the pancake is deep golden brown at the edges and very puffy, about 20-25 minutes. Dust with confectioners’ sugar and serve immediately with cranberry relish.