New York City has the best crumb cake, often sold in individual squares like brownies, with about two inches of crumb topping.  I used to buy hunks every now and again at Mama Joy’s when I was at Columbia.  I was leafing through the May/June 2007 issue of Cook’s Illustrated, which has a decent recipe, though the crumb is not exactly right.  It’s good, but not spot on.  It tastes too much like brown sugar and is floury so I may try to futz with it.  They do, however, give you the technique for producing professional-level crumbs.  The one thing I discovered is that you need to wrap the cake well and let it sit overnight to get the right texture.  If you have a good recipe then please email it to me so I can once again enjoy one of the few cakes I actually like!