My favorite cake is made from 8 egg yolks, leaving me with the whites. Typically, I will make a Swiss-Meringue Buttercream with the whites.
With layers, though, that ends up being a little too much butter a little too rich. What else could I use as a filling that would incorporate the egg whites? I looked up some baked egg-white custard recipes; I've never eaten that so I don't know if, after it baked, it would be creamy enough to use as a filing.
I could just use a thin layer of raspberry jam or something, but I really like the symmetry of using up all the egg whites in the cake, particularly since then I don't throw out the whites because I've forgotten to use them up before they spoil.