At the Fancy Food Show in San Francisco a few months ago, I trudged through the North and South halls of the huge Moscone Convention Center until I was exhausted and almost stupefied by the endless conveniences offered. There must be a baking mix for every occasion. So why bother to bake from scratch? Believe it or not, sometimes it's easier. Certainly, this versatile, tender, one-layer cake is for me â€• I always have the basics: butter, flour, sugar, baking powder, and a couple of eggs.