Combine first 4 ingredients in a medium saucepan; bring to a boil. Cook until sugar is dissolved and mixture is thick (about 3 minutes), stirring frequently. Remove from heat; stir in Kahlúa. Cool.
Combine chocolate mixture and cake in a large bowl; stir in chopped candy bars, reserving 1 tablespoon for topping. Set aside.
Beat 3 cups milk and pudding mix at medium speed of a mixer until well-blended. Stir in cake mixture. Cover; chill 15 minutes.
Spoon half of cake mixture into a trifle dish or bowl. Arrange 1 cup bananas evenly over cake mixture; top with half of whipped topping. Repeat layers; end with whipped topping. Sprinkle with reserved 1 tablespoon chopped candy bar. Chill 1 hour.