streusel (from a German word meaning “to strew” or “to scatter”) is a popular, easy-to-make, sweet crumbly topping for many kinds of baked goods and desserts. The basic ingredients are flour, sugar, and fat, which are combined to form a dry, somewhat lumpy mixture that is usually sprinkled in an even layer on top of a cake batter or yeast dough before baking. In a few modern recipes the flour is replaced by Bisquick (a commercial baking mix of flour, baking powder, salt, and oil), Grape-Nuts cereal, or raw rolled oats. Granulated white sugar is the most common sweetener, but brown sugar and confectioner’s sugar are also used. The fat is usually cold butter, although some recipes use melted butter, vegetable oil, or vegetable shortening. Additional ingredients can include breadcrumbs, chopped nuts, poppy seeds, grated orange and lemon zest, cocoa powder, nut extracts, and spices such as cinnamon, nutmeg, cloves, ginger, allspice, vanilla, and salt. Occasionally a beaten egg is added to help hold the dry mixture together.
