1) Preheat the oven to 350°F. Butter a 9 ½" to 10" (2" deep) cast-iron skillet; or a 9" square cake pan.
2) Combine the apples with the brown sugar, boiled cider, spices, and salt. Set aside.
3) Combine the flour, sugar, baking powder, and salt. Set aside.
4) Mix the warm milk, egg, melted butter, and vanilla. Add to the flour mixture, stirring to combine. Pour into the prepared skillet.
5) Spoon the apple mixture onto the batter. For the best appearance, make sure the apples are distributed a little more heavily towards the edges of the pan. Sprinkle with coarse sparkling sugar, if desired.
6) Bake the cake for about 50 to 60 minutes, till it's light brown and a cake tester inserted into the center comes out clean.
7) Remove from the oven, and cool for about 5 minutes. Loosen the edges of the cake from the pan, and cool for another 20 minutes or so.
8) If you haven't sprinkled with coarse sparkling sugar, dust with confectioners' or glazing sugar, and serve with whipped or iced cream, right from the pan.
Yield: 10" cake, 8 to 10 servings.