Beat cake mix, water and eggs with electric mixer for 1 minute at low speed. Scrape bowl. Continue to beat for 3 more minutes or more on high until smooth. Pour into prepared cake pans, bake according to pkg directions until toothpick tests clean in center or at 325 degrees for 20-25 minutes.
While cake is baking, prepare topping. In top of double boiler, over simmering water, combine butter, chocolate chips, sweetened condensed milk and corn syrup. Cook over simmering water stirring occasionally until smooth, about 20 to 30 minutes. Remove from heat. Beat with electric mixer until smooth.
Frost cake layers.
Variations to this cake: milk chocolate chips instead of semisweet chocolate chips.
Bake cake in 9 X 13 in. pan for 30-35 minutes, cut into squares and pour on topping. Can heat topping in microwave a serving at a time. Stores in refrigerator for a few months if kept tightly closed.