Crumbly sweet toppings are sprinkled as a final layer on top of coffee cakes or muffins before baking or are sometimes used as a filling. Another term for this common topping is streusel, a German word that means sprinkle. Such toppings consist of flour; sugar; sometimes spices, nuts or citrus zest; and butter. The butter, either melted or cut into pieces, is worked into the flour and other ingredients to form coarse crumbs. Streusel toppings bake into a crumbly or crusty texture, depending on the ingredients used.