Mix the sauce ingredients well and set aside. Bring water to a boil in a medium size pot. Add the rice cake pieces. Boil until all the pieces float to the top.
Thinly slice the beef into about 2-inch long strips. Cut stems off the mushrooms and slice into thin strips. Marinate while preparing the other ingredients.
Cut the zucchini in half lengthwise and then thinly slice crosswise and slightly diagonally to make the cuts longer. Generously sprinkle salt over zucchini slices, and set aside for 10 - 15 minutes.
In the same skillet, cook the beef and mushrooms over medium high heat until the meat is cooked through. Reduce the heat to medium low. Add the rice cakes and stir fry for another minute or two. Turn the heat off.
Add the rice cakes to the bowl with the vegetables, along with the optional ginkgo or pine nuts, and the remaining sauce. Toss well to combine everything well. Adjust the seasoning. Serve warm.