Preheat oven to 350°.
Combine first 5 ingredients in a large bowl; beat at medium speed of a mixer until well-blended. Spoon batter into a 13 x 9-inch baking pan coated with cooking spray. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on a wire rack.
Combine milk and pudding mix in a medium bowl; prepare according to package directions.
Tear half of cake into pieces; place in a 3-quart bowl or trifle dish. Pour half of Kahlúa over cake pieces; top with half of pudding, whipped topping, and chocolate bars. Repeat procedure with remaining cake, Kahlúa, pudding, whipped topping, and chocolate bars. Cover; chill at least 4 hours.