I make a smaller version of this all the time.

I just melt 1/2 square chocolate in the microwave with some butter. Then stir in some softened cream cheese, vanilla, and sweetner for a single serving treat. Splenda has always worked fine for me even without powdering (I do use packets not granual please note). The powdering may just make it a tad smoother, but that would be about only benefit I could see.