Wrap the tofu in 2 layers of paper towel. Put a plate on top and leave it to drain for 30 minutes.
To make the sauce, put the dashi, mirin and soy sauce in a saucepan on a medium heat. When the sauce boils, reduce the heat to low. In a small bowl, mix the potato starch with the water to make a smooth paste. Add to the sauce and keep stirring until it thickens, then remove from the heat.