Layers. Not merely layers in the cake, but layers in the photo itself. See what I did there… Yeah, wordplay can be fun.
In one frame this is a detail shot of the cake, a story shot of the guest signing the book, and a setting shot that helps to establish the layout and overall “feel” of that particular part of the day. By keeping the background slightly out of focus we put the primary emphasis on the cake, but we see it in a larger context than if it were a glamour shot all by itself.