I use heirloom or cooked flour buttercream. It can be a little temperamental (it'll get pretty soft and even separate if it get's just too warm). When I cover it in fondant I do chill the cakes so that they are firm and will hold their shape. However I do deal with condensation, and often the frosting gets very soft and before my cake is finished it has often gone in and out of the fridge several times- I find I need to re-chill it or else I end up with indents and frosting sliding around under the fondant!

With this, I sometimes find that when the cake is cut the frosting, especially on the outer edge that touches the fondant, has separated somewhat and I notice it seems like the frosting was maybe kind of melting. I'm thinking 2 things are happening: 1, the changes in temperature are ruining the frosting's structure/texture, and/or 2, the condensation is actually getting the buttercream wet, and ruining it. 

I am working on perfecting this method because I love this frosting and so do the people who eat my cakes, and sometimes a cake just needs fondant to achieve the right look. I know I may have to sacrifice this pairing, but does anyone have any tips/thoughts/suggestions/experience with this? 

Have you considered using this frosting as a filling and using a more stable icing as your outer edge?  You could even use an apricot jam as your outer icing for the fondant?  I've had the cooked flour icing before and it is very yummy, but I could also see where it would be a bit temperamental.  You also might try using a high ratio shortening in place of the butter?  Hope you figure it out :)

Thanks so much for answering! I know I might have to sacrifice it for something else, but it is such a hit and my Nana's recipe, so it is dear to me and I want to try to keep using it.

Why apricot jam? I haven't heard of this.

Shortening is a good idea- maybe I'll try a combination of that and butter just for the outside portion. I recently made a SMBC but didn't love it- I know I need to make it a few more times to get the texture right, but I did find it to be too much like plain butter in texture and taste, which I didn't like. 

The apricot jam isn't high in flavor, but still adds the sweetness.  Plus it's nice and sticky so the fondant will stick well.  I'm partial to my recipes as well, so I completely understand :)

I see… I may have to try that on the right cake. Thanks for that tip!