I would half the butter in the recipe and add shortening since it won't melt as easily as butter.
I agree with the sentiment that switching to an all shortening buttercream will help. Butter has a melting point that can start as low 88* whereas Crisco has a melting point of 106*. Some of the commercial shortenings have melting points over 115*.
You may want to add a little extra butter flavoring to help the taste a bit (I don't recommend 'Flavor Mill' Butter Flavor unless you like the taste of Butterscotch
)
Best of luck!
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