A good crumb bun is hard to find. Tipped off by a friend who is a devotee of this homey coffee cake, I tracked down Michael and Wendy London at Rock Hill Bakehouse in Gansevoort, New York. They are pioneer artisan bakers, who produce some of this country’s best wood-oven-baked breads. Their crumb buns are light-years from the commercial variety; the yeast dough, rich with sweet butter and sour cream, is showered with plenty of crumbly streusel. Crumb buns—a