I didn't know what American Frosting was before I made this cake, and after baking it and eating a decent amount of it, I still have no idea. What I can say for certain is that, despite following the instructions exactly, the rather grainy paste that I ended up smearing all over the sponge is not it! I hope not anyway, as it was exceedingly disappointing, disturbingly crunchy and too sweet, even for me, which is saying something indeed!
The shame of it is that the cake itself was absolutely delicious. I'm not a massive fan of walnuts so I substituted pecans in what I think was a master stroke. I will definitely be making it again, although I'll have to sort out what happened with the frosting. This is the problem with combining recipe books and novices. Mary Berry clearly thought that anybody would be able to follow her instructions and produce a decent version of the frosting. I can always be relied upon to confound such expectations.
As to what this recipe was doing in the middle of the 'fruit cakes' section . . . well, maybe that's a question for another day!