I found the easiest way to grate the chocolate bar is by chopping it with a large knife. It will just shave right off.
Note from my sister-in-law: This has been a family favorite for as long as I can remember. However, Marc's introduction to this yummy dessert has become a funny story that I must share. Shortly after Marc and I started dating, my mom packaged up several slices of this yummy cake for me to take back to my dorm room at the University of Arkansas. She sent enough for me to share with Marc (this would be his first time to try my mom's cake). As soon as I got back to school that Sunday afternoon, my mom called to ask if we'd eaten any of the cake yet. She was in a panic because she had just realized that one of her blender blades was broken and missing! (She'd used the blender to chop the German chocolate bar!). So we carefully ate our cake, and sure enough, Marc's piece contained the missing blender blade! Who knew at that point that Marc would be her future son-in-law, but we now joke that she was trying to scare him off early!