Flour, Sugar, Butter, Eggs. The basic building blocks of most cakes and pastries. The infinite variations, combined with a few flavorful additions, never cease to amaze me. Try this recipe for a Kirsch Streusel Kuchen, a German Cherry Streusel Cake with both chocolate and vanilla Streusel. Those four basic ingredients, with the addition of a few tablespoons of cocoa, some sour…