The background: Fondant originated in Europe where it was used to cover fruitcakes to keep them fresh for shipping to the New World.
The difference: Fruitcakes are not torted (cut horizontally) or filled with anything. Also, fruitcakes are traditionally covered with a 1/4″ layer of marzipan and then covered in fondant.
The result: Perfectly smooth and gorgeous fondant.
The problem: American cakes are torted, filled, iced, and then covered with fondant. Often, the filling softens, and the weight of the fondant causes it to smoosh out. (If you’ve ever had this happen, you know that “smoosh” is the only applicable description.) Maybe your crumb coat wasn’t thick enough, or the cake wasn’t leveled evenly, or you didn’t refrigerate it long enough. Your cake, which started off looking beautifully smooth, ends up looking unprofessional, like the cake below created by some anonymous cake designer. (Okay, it was me. But it was like a really long time ago.)
The solution: Cake spackle! Toba Garrett, a master cake decorator (you should see her string work!) and instructor at The Institute for Culinary Education developed this technique, which gives the cake “more stability and structure” so that layers are not visible through the fondant. I found it in her book, The Well Decorated Cake, and have been using it for years.
First, level your cake. Save the domed portion that you leveled off and usually throw away for the next step!
Fill the cake–but don’t crumb coat–and refrigerate until firm.
Take the domed portion and crumble it up. (I use my hands, but a food processor would work too.)
Mix the cake crumbs with buttercream and some filling until it is thick and pasty. (I would recommend about a 3:1 ratio of cake crumbs to buttercream.) This is your cake spackle!
Now ice your cake with the spackle and refrigerate until firm. If the spackle is too thick to spread, +–some more buttercream.