Meanwhile, prepare the sauce: mix the soy sauce, the brown sugar, the sesame, the water, the rice vinegar, the chives if you have decided to use them, the freshly grated ginger, and 1 1/2 tablespoons of potato starch. Mix well and put over tofu in the skillet. The trick here is to try the sauce as you mix it. You can personalize until you reach the right level of sweetness, spicy, salty and tangy. There is really no right way other than slowly adding the ingredients and decide, with your own taste, if it does transport you into a Chinese restaurant. If the answer is yes, then the sauce is ready. Trust your instincts!