Another one from Alton Brown.
Ingredients
4 to 5 cups dashi, recipe follows
2 tablespoons brown miso paste, plus more to taste
2 tablespoons white miso paste, plus more to taste
6 ounces firm tofu, cut into 1/2-inch cubes
2 scallions, white and green, thinly sliced on the diagonal
2 tablespoons aji mirin (sweetened rice wine), optional
Directions
In a saucepan heat the dashi and whisk in the miso pastes. Bring to a simmer and add the tofu, scallions, and mirin, if using. Remove from the heat and serve immediately.

DASHI
6 cups cold water
One 12-inch long piece of kombu, wiped with a damp cloth
One .88-ounce/25 grams package shaved dried bonito flakes
In a saucepan, combine the water and kombu. Bring to a simmer, uncovered, over medium heat. Remove the kombu and discard. Bring the liquid to a boil, add the bonito flakes and immediately remove the pan from the heat. Allow the dashi to sit undisturbed for 2 minutes. Strain through a fine mesh strainer into a medium bowl. Discard the bonito flakes. Use immediately or store, covered, in the refrigerator for up to 3 days.

Yield about 4 to 5 cups