There is a wonderful poetry to the name of this pudding which, thankfully, once eaten could never be forgotten. It's an old, old recipe popularly exhumed - I believe - by the late, great Richard Sax. Think of it as a kind of marshmallow-based pavlova. That's to say, you whip egg whites as if making meringue, spread on a swiss roll tin (or that's how I make it) and put in an oven which you immediately switch off, leaving the pudding to cook overnight, hence "forgotten".
Recipe posted by Nigella