Preheat oven to 350°F. On stovetop, in bottom of 10-inch cast iron or heavy skillet with ovenproof handle, melt butter. Sprinkle evenly with brown sugar.
Arrange Pineapple rings in single layer over sugar in skillet (if necessary, cut 1 or 2 slices into pieces to fit into spaces and holes).
In large bowl, combine flour, granulated sugar, cornmeal, baking powder, xantham gum, cinnamon, and salt with wire whisk. In small bowl, whisk together the milk, oil, egg, and vanilla, until blended. With mixer on low speed, add milk mixture, reserved pineapple juice and cider vinegar to flour mixture and beat until blended. Increase speed to medium and beat 1 minute. Spoon batter over pineapple and spread evenly.
Bake 35 to 40 minutes, or until toothpick inserted in center comes out clean. The top should be very golden brown. Cool in skillet on wire rack 10 minutes. Run knife around side of cake to loosen from skillet. Place platter on top of skillet; carefully, but in one quick motion, invert cake onto platter. Remove skillet. Let cool 30 minutes. Serve warm or cold plain, with whipped cream, or sour cream. Makes 16 servings.
