This apple upside down cake has a glaze of caramelized apple slices atop a moist spiced yellow cake.
Preheat oven to 350°F. Place 1/2 cup (1 stick) of the butter and brown sugar in small saucepan. Cook on medium heat 2 minutes or until sugar is dissolved. Stir in 2 teaspoons of the vanilla and 1 teaspoon of the apple pie spice. Pour into 13x9-inch baking pan. Place apple slices in even layer in bottom of pan. Set aside.
Beat cake mix, eggs, sour cream, remaining 1/2 cup (1 stick) butter, 2 teaspoons vanilla and 2 teaspoons apple pie spice in large bowl with electric mixer on low speed 30 seconds or until moistened. Beat on medium speed 2 minutes. Pour batter over apples.
Bake 25 to 28 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes. Invert onto serving platter. Spoon any remaining sauce over apples. Serve with whipped cream, if desired.