I always have a problem with the jam/jelly getting mixed with the buttercream.  As a filling, it doesn't matter much, but when under the frosting on top, that creates a problem.

I have always planned on experimenting, but I have run out of time.  Here is what I want to do:
take wax paper and line my pan (whatever size the cake is)

Melt the filling (I like the the Polaner seedless raspberry fruit spread)

Pour thin layer it into the lined pan and freeze

When it is time to frost the cake, take it out and place it on the cake just before applying the buttercream.

It seems like such a simple solution.  The frosting should go on without the the jam getting mixed in....plus the added bonus of having a uniform thickness to the jam.

but if it were that simple, everyone would be doing it (I'm sure I can't be the first person who has thought of it), but I can't seem to find anything on the subject.  So can someone tell me if this will work or not?

thanks!

Maybe its just me but I only use raspberry jam in the filling & not under the frosting on the outside. If its purely for flavour you could use boiled jam and brush it on, it will soak into the cake a little more and maybe then won't mix with the frosting as much, but if it were me I wouldn't use the jam on the outside

I've never used jam under buttercream on the outside of a cake either - sounds messy!  Where did you learn to do this?

Liz
 

Most of the raspberry cakes I have had have been like that.  I do remember - many years ago - having a problem finding a person to do my wedding cake with raspberry....many either wanted to use just raspberry as the filling or mix the raspberry with the buttercream (YUCK).  But I finally found one of the better cake artists in town and they did it for me.  The top of each tier also had the raspberry under the buttercream.  The raspberry filled cakes I have had recently have also had a layer on top under the buttercream. 

I still haven't figured out how he did it though.  when I do my inner layers, I just spread on the raspberry on one cake and ice the bottom of the next cake with buttercream and then place it on top.  Of course that won't work on the top of the cake...so I am trying to figure a way to make it work.

Well, I've never in my life seen a cake with filling "on the top" and a layer of BC over that.  I've also never heard of freezing a jam/preserves into a layer and putting it in, or on, a cake.

For the "top" issue, I'd guess that the raspberry was done like a gelee/pate de fruit so that it was very sturdy and therefore, cut-able.  I wouldn't do it--too much work--but it could be done.  I also wouldn't want the consistency of a gelee as my filling--YUCK!

As for freezing jams/preserves--the structure of the product has been changed by freezing it.  I'd guess that when it defrosts, it's way more likely to weep out lots of water.  Freezing causes any fruit particles to "explode" and become more macerated--this equals even more moisture than the pectin or gelatin in the product can handle.

When I use raspberry preserves or fruit spread, I butter the cake layers with BC, pipe a good dam of BC, spread on the preserves, stack the layers, and have no issues with the preserves seeping into the cakes.

I still think freezing the jam would cause problems, it would create liquid as it defrosts which would seep into the cake and would likely also affect any buttercream applied on top. I've thought of a method that could possibly work, perhaps try chilling the cake tin you were intending to use, lining with greaseproof / wax paper, pour boiled jam into the chilled & lined tin and allow it to set again. Rather than actually freezing the jam. This should produce a jam disc the dimensions of the top of the cake that you could apply buttercream on top of, also rather than spreading buttercream you could try piping on a layer first over the jam and smoothing rather than actually spreading over the jam. Just some suggestions, it isn't something I'd personally do or have done.