Grease and flour one 9-inch round baking pan; set aside. Prepare cake batter as directed on package. Pour half of the batter into prepared pan; spoon remaining batter evenly into 12 paper-lined medium muffin cups. Bake as directed on package for 9-inch cake layer and cupcakes. Cool cake and cupcakes in pans 5 min.; remove to wire racks. Cool completely.