I'm going to try the microwave method and see what happens. I hope it works b/c I need it for another experiement I'm toying with. When I worked at Fresh Fields, we had a chocolate cake with pumpkin buttercream for Halloween. Then used a Halloween themed stencil and dusted cocoa powder over it. It was a big seller! All we had to do was add pumpkin spice and canned pumpkin to vanilla Italian meringue buttercream (or was it Swiss?, eh, who can remember... hmmm, no wait, it was Swiss) until "it tasted right". Or in my case, I don't eat eggs, "until it looked right". I tried to make it with American buttercream, using powdered sugar, butter, vanilla and salt, but American buttercream just does't work that way. You add the slightest thing that's not in the original recipe and it throws the whole thing off. It breaks when you add too much pumpkin. But if you don't put enough, you can't taste it. I tried eliminating some of the butter, but that didn't work either. After lots of trial and error, (adding starch, flours, etc.) it worked out and it was so **** good! But I was thinking to use less pumpkin puree and add some powdered pumpkin to get more flavor without changing the consistancy.
If any of your like pumpkin as much as I do, add it to vanilla buttercream with some pumpkin spice. Nummy! You'll love it!