I prefer to soak my konbu for 24 hours to make my dashi, but there are those times where one forgets to do so and a dashi is needed. This method does take sometime, about 1 to 1.5 hours to make, but produces a great dashi. For this particular dashi I add dried shiitake to increase the umami.
1/2 oz konbu
1/2 oz Shiitake, dried
4 cups water
3/4 oz katsuo bushi
In a medium-sized pot, soak the konbu and shiitake in the water at room temperature for at least 30 minutes, and up to 1 hour (the longer the better).