The first thing you need to know about streusel is that it's different from those delightful, Germanic, often-sweet-but-sometimes-savory filled flaky pastries known as strudel. Streusel, also a delight, is not a standalone pastry but rather a crumbly-sweet topping that can adorn pies, muffins, and, yes, coffee cake. A simple concoction at heart, streusel is made by combining butter, flour, sugar, and sometimes spices, then sprinkling that mixture on top of (or occasionally inside) whatever you're baking just before popping it in the oven. Streusel topping is sweet, buttery, and crumbly — the perfect flavor and textural complement to the light, sweetly-spiced, yeasted bake we know as coffee cake.
American coffee cakes are the legitimate heirs to a dessert called guglhupf, a ring cake probably invented by the French but definitely claimed by the Austrians. There is no doubt that the (clearly brilliant) habit of drinking coffee while enjoying guglhupf is entirely Viennese — and, like any good idea, it caught on in the New World. Streusel is also of Germanic origin, with a starring role in streuselkuchen, a flat crumb cake. Ultimately, was a culinary and geographic inevitability.
The German word "streusel" means to scatter or sprinkle, and in this case, name of the thing is also what you're supposed to do with it. What could be more sprinkle-able than a crumbly confederation of butter, sugar, and flour? Like most things that look easy, there's a slight trick to making streusel, and it revolves around butter. Namely, it's easiest to handle when brought up to room temperature. (One strategy for accomplishing this in a hurry is to cut the cold butter into small pieces.) Then it's just a matter of mixing and sprinkling.
This approach is so basic that it lends itself to almost endless variation. You can, for instance, substitute brown for granulated sugar. You can add oats to enhance the overall flavor and crunch, or toss in some slivered almonds or small walnut pieces. The inclusion of ground cinnamon is also quite common, and if you let your imagination run a little bit this could also extend to other sweet baking spices like grated nutmeg, ground cloves, and cardamom seed. Can't get enough of pumpkin spice? Streusel's got you, fam.
If you think there's a wide variety of streusel recipes, wait till you hear about potential ways you can make streusel coffee cake. How about one filled with juicy blueberries and crunchy pecans, or one made from graham cracker crumbs? For a textural shift, try an autumnal coffee cake, made tender with apples and pears, or the exquisite softness of a coffee cake made with sour cream topped with crunchy streusel. During the holiday season, a on the breakfast table will never last long.
Once you get into streusel, you'll find it also works equally well sprinkled on other kinds of cakes, as well as muffins, fruit tarts, cupcakes, pies, cobblers, or even bread. Consider altering your basic streusel recipe to better complement your bake; for example, you can add desiccated coconut to your streusel topping for a coconut coffee cake. Whatever you do to get there, it's all about that moment when you sit down with a steaming cup of deep, dark coffee and a slice of sweet, crumb-topped cake.