Orange Upside-Down Cake

Preheat oven to 350 degrees F. In bottom of 10-inch cast iron or heavy skillet with ovenproof handle, spread butter. Sprinkle evenly with brown sugar. From oranges, grate 1 teaspoon peel; set aside. Cut peel and pith from oranges; discard. Slice oranges crosswise into 1/2-inch-thick slices; arrange in single layer over sugar in skillet (if necessary, cut 1 or 2 slices into pieces to fit into spaces). In large bowl, with wire whisk, combine flour, granulated sugar, cornmeal, baking powder, and salt. In small bowl, whisk milk, oil, egg, vanilla, and orange peel until blended. With mixer on low speed, add milk mixture to flour mixture and beat until blended. Increase speed to medium and beat 1 minute. Spoon batter over oranges and spread evenly. Bake 35 to 40 minutes, or until toothpick inserted in center comes out clean. 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 invert cake onto platter. Remove skillet. Let cool 30 minutes.