I've bee having some problems with buttercream melting, how can I stabilize it without resorting to subbing in some vegetable fat or increasing the sugar content?

I imagine gelatin would work but it needs to be vegetarian, I tried agar but it either melted the butter or solidified before being fully combined.

Another option I'm yet to try but I image would work is adding melted white chocolate, at a ratio of 1:1 or 2:1, on the basis that ganache is pretty temp stable.

I tried adding cornstarch but that did absolutely nothing.

Guar gum is coming in the post so that's my next step. 

Any further suggestions?

add more cornstarch -- it works

also cut down on your liquid -- other than that get the cake and icing out of the heat