Prepare cake according to package directions. Set aside half of batter; pour remaining batter into a greased and floured 13x9-in. baking pan.
Bake at 350° for 18 minutes. Meanwhile, in a small saucepan, melt the caramels, milk and butter. Remove from heat; stir in nuts. Pour over cake. Sprinkle with chocolate chips, then pour reserved batter over top.
Bake 20 minutes longer or until cake springs back when touched lightly. Cool on a wire rack. Cut into squares and top each with a pecan half if desired.