The three basic ingredients for the crumb topping in an old-fashioned crumb cake are butter, flour and sugar. Bakers can add other ingredients, such as vanilla extract and cinnamon, to the crumb topping to give it more flavor.
Though variations exist, recipes for the crumb topping generally call for one part sugar to one part butter to two parts flour. If the baker substitutes margarine or shortening for the butter, the consistency of the crumb topping remains the same, but the flavor may be altered. After mixing together the ingredients, bakers sprinkle the crumb topping over cake batter and bake it.