I have been testing additives to buttercream to make it more stable in high temperatures.

At the same time, I borrowed Toba Garret's Professional Cake Decorating, second edition from the library, and was comparing it to my copy of the first edition. The great lightbulb went on

Chef Garrett has certainly developed one solution to the issue of making a stable undercoat for findant coverings: her Spackle Paste. This goes on over the sheer crumbcoat of buttercream, is stable and tasty. The cakes shown in the book have been filled with buttercream (fruit with a buttercream dam).

Garrett Spackle Paste
20 ounces fresh cake crumbs (567 grams)
4 ounces fruit curd or fruit preserves (114 grams)
6-8 ounces any type of buttercream (180-230 grams)
Buttercream may be regular, crusting, custard, all types of meringue

Method
1. Mix cake crumbs, fruit, and 6 ounces of buttercream. Mix well and add more buttercream ONLY to make a stiff paste that looks like bread stuffing. This mixture must be used at room temperature. Chef Garrett doesnt note any method for premixing, chilling, and warming up the paste.

2. Place 8-10 ounces (230-280 grams) of this stiff paste on a plate/cake cardboard and 4-6 ounces buttercream next to this. Beat the paste to soften it, and then add enough buttercream as required to make the paste spread easily onto the cake.

3. Apply the paste with a metal spatula to the crumb coated cake, making a 1/8" layer. Use the extra buttercream on the patula to help spread the paste onto the cake--but dont mix it into the bulk of the paste except if necessary to make the paste spread well.

4. Smooth the paste the same way as you would smooth a thin layer of buttercream icing (water is OK). Add one more thin layer of paste and smooth it, for a total of just under 1/4" thick layer at the end of the smoothing process. The top should be flat, and the edges can be square or rounded according to the desired final finish.

5. Chill the paste-coated cake until firm. Just before covering with rolled fondant, add a very very thin layer of soft buttercream to help the fondant adhere to the cake.

What isnt discussed: no mention of what to do for chocolate flavour. I suspect that you could use 2 ounces of chocolate melted with 3 ounces cream in place of the fruit curd, but then you might need to adjust the consistency with more buttercream.

FYI having looked over both editions, there are 50 pages more in #2, and the cake gallery is different photos of the same examples. The decorated cookies and modelled figures appear to be the biggest changes.

Wow, an interesting phenomenon. I'm going to read over your post again. This would be fun to try. Have you tried it yet?

This is interesting. Let us know when you try it and post pics if you can.

I don't do fondant cakes. Primarily because I have been schooled to believe that a cake should be chilled until half an hour before serving...winter or summer.

I have baked/decorated more tiered and single layer cakes with buttercream than I could possibly count. I have made a specialty of European style nut flour cake layers with dacquoise layers and custard based buttercreams. I used poured ganache sometimes on the outside. People always asked for "not too sweet" cakes and loved what they got. I even made one tiered cake this way for a small wedding, it was stable because it was chilled all the time.

I have put fondant onto fruitcakes on top of a layer of marzipan. I made my own marzipan from scratch last winter and will continue to make it that way (will post recipe this week). The almond lovers like the heavy undercoat and the thinnest topcoat for decorating that I can manage.

I have seen questions on this forum that I consider to be due to the use of convenience materials. For example, egg white royal icing dries hard as a rock when mixed according to the classic British method, and doesn't fall apart when stuck into buttercream. But use meringue powder and whip it, and you will have royal icing that just doesn't hold up on buttercream...