Position a rack in the lower third of the oven and preheat to 220°C/425°F/gas 7. In a large frying pan, melt 2 tablespoons of the butter over medium-high heat. Add the apples and brown sugar and cook, turning as needed, just until crisp-tender, about 4 minutes. Sprinkle with the cinnamon and stir to combine. Add the remaining tablespoon of butter and swirl the pan until the butter melts and coats the bottom and sides of the pan.In a bowl, whisk together the eggs, milk and vanilla. Sift the flour and salt over the mixture and whisk until blended. Carefully pour the batter over the hot fruit. Bake until puffed and golden brown, 20-25 minutes. Dust the top with confectioners’ sugar.