Ok, the butter helped some also. And I remembered I have some chocolate extract in the pantry I've had for years and I think that helped a huge amount. I think the alcohol in the extract does something with the chocolate or splenda to make them mix together better.

It now as a moderate chocolate bitter taste followed by a very sweet splenda taste, but it isn't as awful as it was. In fact, if you like dark chocolate, and I do, this might appeal to you.