Nutella! OH YUM!
I also have not noticed a bitter aftertaste, and have made a number of batches of the recipe, so it may be your vanilla. I have also added a bit of candy oils. Cheesecake, cherry, coffee and pina colada. YUM! (I had some leftover icing and was just testing out the flavorings to sample before endeavoring it on a cake lol!)
Another thing I like about the Dream Whip recipe is that one cake I made I then covered with a cake dome. Which of course seals in moisture. The icing 2 days later was very glossy and pretty-sort of like a meringue BC. But it still held the shapes and didnt "melt" or anything. Just looked really pretty.
I will try the chocolate cocoa variation this weekend. I like using ganache but covering a cake in ganache and filling it with ganach it ends up being verrrrryyyyy rich. I used a lot of ganache in my miniature pastries and a few "undecorated" cakes just garnished with chocolate curls and pirouettes (sp?). So I was looking for a lighter chocolate icing to use with ganache filling. I will try the Dream Whip in chocolate.
A big note: DO NOT use the cheapie Dream Whip substitutes. There are a few brands. I tried one. YUCK-it tasted like flour and cornstarch! ONLY Dream Whip brand.