Streusel is the German word for "something sprinkled or strewn." In bakeries all over the world, it is the term commonly applied to a type of crumb topping that is liberally spread over any number of baked goods. Traditionally made from a combination of fat, sugar and flour, modern variations sometimes include nuts, brown sugar and cinnamon or other spices. The more economical home baker will frequently substitute shortening or margarine for butter, as well. Such is the popularity of streusel topping that it can be found gracing everything from doughnuts to muffins to Dutch apple pie.
Combine flour, sugar and salt in a bowl. For each cup of flour, use ¾ cup granulated sugar and ¼ teaspoon salt. Then, for each cup of flour you've used, cut in 6 tablespoons of chilled unsalted butter, 1 tablespoon at a time. Cut the butter into the flour mixture with a fork or pastry cutter until crumbs begin to form.
Add any of the following to the recipe: ½ teaspoon of ground nutmeg, 1 teaspoon of ground cinnamon, 1 teaspoon of vanilla extract, 1 cup of chopped walnuts, or 1 cup of chopped pecans, if desired.
Sprinkle the crumbs over unbaked muffins, on top of banana bread, across cake batter or use instead of a top crust on fruit pies. Bake as usual.
Store any unused streusel crumbs in the freezer. Using your hands, press the remaining crumbs together to form one large lump and then place the lump into a plastic freezer bag. Seal the bag and place into the freezer. Streusel will keep for up to six months.