This beauty has all the beloved flavors of the classic Black Forest cake — cherries, chocolate and cream — re-assembled into a casual sheet cake perfect for summer picnics and birthday parties. To make the process even simpler, you can make and refrigerate the cherry jam up to 3 days in advance. Feel free to use fresh or frozen fruit (and there’s no need to thaw first, if using frozen). You can make this cake in the summertime, when fresh cherries are at their peak, or in the dead of winter…