Beat butter at medium speed of an electric mixer until creamy. Gradually add sugar, beating until light and fluffy. Add egg and molasses, mixing well.
Combine flour and next 5 ingredients; add to butter mixture alternately with water, beginning and ending with flour mixture. Beat at low speed after each addition until blended. Pour batter into a greased 13" x 9" pan.
Bake at 350° for 30 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack.
Cut gingerbread into cubes. Arrange one-third of gingerbread cubes in a 3-quart trifle bowl; top with one-third of Custard. Repeat layers twice, ending with Custard. Cover and chill until ready to serve.
Before serving, spread whipped topping over trifle. Sprinkle with chopped candy bars.