Combine and mix well cake mix, butter, water, oil and half the can sweetened condensed milk. Pour half of the batter into a greased and floured 13- by 9- by 2-inch baking pan. Bake in a preheated 350° oven for 20 to 25 minutes.
In a saucepan over low heat, combine caramels and remaining half can condensed milk. Stir mixture, heating until caramels are melted and mixture is smooth. Spread over the baked cake layer. Sprinkle generously with chopped pecans. Cover with remaining cake batter. Bake 25 to 30 minutes longer. Frost with Turtle Cake Frosting.
Turtle Cake Frosting:
Cream butter, add cocoa and powdered sugar alternately with half-and-half. Add vanilla and corn syrup.