HEAT oven to 350 degrees F. Coat 13x9-inch pan lightly with no-stick cooking spray.
PREPARE cake mix according to package directions. Pour 2 cups batter into prepared pan. Bake 15 minutes.
MELT caramels and butter in heavy saucepan over low heat. Remove from heat. Stir in sweetened condensed milk until blended. Spread caramel mixture evenly over cake. Spread remaining cake batter over caramel mixture.
BAKE an additional 25 minutes or until cake springs back when lightly touched. Cool. Mix caramel topping and pecans in small bowl. Spread over cooled cake.