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Beat them together until your mixture is nice and smooth. You may find that the butter isn't absorbing into the cream cheese easily--just keep mixing. If you use a low-fat version of either, the two may never fully combine. Though it's better for your waistline and health, higher-fat products will yield better looking results. When the egg is well mixed in, add 1 tsp (5 g) vanilla. Don't beat any more than you have to or the mixture might grow thin. Beat it slowly so that you don't make a mess. You'll know when you're done where there are no more lumps. If the lumps seem to stay and you don't want to over beat your batter, take the back of the spoon and crush the lumps against the wall of the bowl. This will firm up the dough so you can roll it into balls more easily. You may consider putting it in the freezer if your tummy is rumbling, but good (better) things come to those who wait.

Summary:
Mix the cream cheese and butter in a large mixing bowl. Beat in the egg. Add the chocolate cake mix to the bowl. Cover the bowl with plastic wrap and refrigerate it for 2 hours.