Cinnamon-sugar crumble does double duty for this classic cake, serving as both a topping and a thick ribbon running through the middle. This coffee cake is light and fluffy, thanks to a cup of buttermilk in the batter. For added crunch, add a cup of finely chopped walnuts or pecans to the crumble mi xture. The cake will keep at room temperature, wrapped well, for several days, but the crumb topping will be the most crisp on the day the cake is made.Yield: One 9-by-13-inch cake (12 to 16 servings) ...read more read less