Prepare the cake according to the package directions for a 9 by 13-inch cake. Cool thoroughly.
Prick the entire top of the cake with a fork. Pour coffee liqueur over the cake and allow it to soak in. Wrap the cake with plastic and refrigerate for at least 3 hours. Slice the cake into 1-inch thick pieces.
Line the bottom of a large glass bowl with a single layer of cake. Pour 1/4 of the fudge sauce over the cake, top with 1/4 of the candy bar-bits then 1/3 of the whipped cream. Repeat layering 2 more times ending with fudge sauce and candy bits. Refrigerate until ready to serve.
To make the sauce: Melt the German chocolate with the butter in a saucepan over very low heat. Stir in the powdered sugar, alternating with evaporated milk and blending well. Stirring constantly, bring the mixture to a simmer over medium heat. Simmer until the mixture becomes thick and creamy, about 8 minutes. Stir in the vanilla.
Assemble the trifle shortly before serving or it will get soggy.