Prepare and bake cake mix according to package directions for cupcakes., In a large skillet, heat butter over medium heat. Add apples and brown sugar; cook and stir until apples are tender, 10-12 minutes. In a small bowl, mix cornstarch and water until smooth; stir into pan. Bring to a boil; cook and stir until thickened, 1-2 minutes. Remove from heat; cool completely., Using a paring knife, cut a 1-in.-wide cone-shaped piece from top of each cupcake; discard removed portion. Fill cavity with apple mixture., In a large bowl, combine the 5 frosting ingredients; beat until smooth. Frost cupcakes. If desired, top with apple slices to serve.