Drain the tofu and cut each block into 4 to 6 large cubes, slicing gently with a slow sawing motion so the silken tofu does not crack.
Wipe the kombu with a damp cloth and place it in a medium pot or donabe with the 4 cups of water. Set over medium heat and bring up slowly until tiny bubbles form around the edges of the pot, about 5 to 7 minutes.
Let the kombu steep at that gentle heat for 10 minutes, then remove and discard it. Stir in the soy sauce and salt.
Slip the tofu cubes into the broth in a single layer. Adjust the heat so the surface just trembles, with no rolling bubbles.
Simmer gently for 8 to 10 minutes, until the tofu is warmed through and has developed a soft sheen. Do not stir.
Divide the ponzu among 4 small dipping bowls and top each with scallions, grated ginger, and a pinch of togarashi if using.
Bring the pot to the table. Lift out the tofu cubes with a slotted spoon, set 2 or 3 into each person's dipping bowl, and pour a little warm broth over. Serve immediately with steamed rice.