Prepare cake according to package directions. Pour into a 9 x 13-inch pan. Cool thoroughly. Pierce holes over the entire top of the cake with a fork. Pour coffee liqueur over the cake, allowing it to soak in. Cover with plastic wrap and refrigerate at least 3 hours. In a glass trifle bowl, layer ingredients beginning with a one-inch layer of cake, 1/4 of the fudge sauce, 1 candy bar, and 1/4 of the whipped cream. Repeat layering 2 more times ending with candy bar and fudge sauce. Refrigerate until ready to serve. Left over cake can be frozen.
Cook's Note: Assemble the trifle shortly before serving or it will get soggy.
Hot Fudge Sauce:
Melt chocolate and butter in saucepan over very low heat. Stir in powdered sugar, alternating with evaporated milk, blending well. Bring to a boil over medium heat, stirring constantly. Cook and stir until mixture becomes thick and creamy, about 8 minutes. Stir in vanilla; serve warm.