Preheat oven to 425 degrees. In a blender, combine milk, eggs, flour, cocoa powder, salt, vanilla, and granulated sugar. Blend until foamy, 1 minute. In a large cast-iron skillet, melt butter over medium. Pour batter into skillet and immediately transfer to oven. Bake until puffed and set, 20 minutes. Dust with confectioners' sugar and serve immediately.