I have a cake to make and the customer wants it to be fondant covered with a less sweet buttercream. I thought about using Rich's Bettercreme as the filling and underneath the fondant but not sure how it will work since it is so thin. Has anyone tried this?
No.. it will melt your fondant.
Is that because of the water content in it? What about if I use it as a filling and put something else underneath the fondant?
Yes it's the water content. You can fill with it then use a standard buttercream (SMBC is far "less sweet") to cover (it won't melt that) then fondant over that.