Slice the block of tofu in half one way, and then in half the other way to make 4 cubes. Sprinkle all sides of the tofu with the salt and then place the tofu on a wire-mesh tray or cooking rack to drain for 20-30 minutes. This step is very important as excess water will make the oil spatter when you fry it.
While you wait for the tofu to drain, season the dashi, by putting the dashi, soy sauce, sugar and salt into a small saucepan and heating the mixture until the salt dissolves. Cover to prevent evaporation and keep it warm over low heat on a back burner.
Grate the daikon using the rasp side of box grater (or a dedicated daikon grater if you have one), and then pour it into a small wire-mesh sieve, pressing on the solids to drain off any excess liquid.
When you're ready to fry the tofu, add 1 1/2 to 2 inches of vegetable oil into a heavy bottomed pot that's large enough to accommodate all the tofu and preheat to 360 degrees F (180 C). Prepare a wire rack lined with a triple layer of paper towels to transfer the tofu to when it's done frying. Add the potato starch to a plate.
When your oil has reached the correct temperature, dry the tofu off thoroughly using paper towels, and then roll each block of tofu in the potato starch to coat every side with a thin even layer of starch. Dust off the excess starch and gently lower each block of tofu into the oil.
Fry the tofu until the sides have crisped and then use chopsticks or tongs to gently roll each piece of tofu over onto the other side. The tofu is done when all sides are crisp and the tofu looks slightly puffy. It should not be browned.
Transfer to the prepared rack and let it drain for a few moments before plating each block of tofu into a small bowl.
Top with the katsuobushi, daikon, and scallions and then pour the hot dashi around the tofu.