Combine cake mix ingredients as listed on box. Beat on medium speed until well combined.
Bake cake in 9-by-13-inch pan for 20-22 minutes. Check doneness by inserting toothpick into center of cake. If toothpick is clean, cake is done. Allow to cool 10 minutes.
While cake is still warm, use end of wooden spoon or another round object and poke holes all over top of cake.
Heat jar of hot fudge sauce in microwave for about 30 seconds, or until you can stir and thin it out. You may need an additional 30 seconds. Pour sauce on top of cake and allow it to absorb. Let set until completely cooled.
Open can of fruit filling and pour over top of cake; spread evenly.
To prepare whipped cream, put bowl and whisk attachment in freezer for 5-10 minutes to get them really cold. In cold mixing bowl, pour in heavy whipping cream and beat on medium speed about 3 minutes. Add powdered sugar and increase speed to medium high. Beat until stiff peaks form. Spread whipped cream over top of cake.
Chop chocolate bar into small pieces and sprinkle over top of cake. Keep refrigerated until ready to serve.