1. Place the mushrooms in a clean heatproof jar with a tight-fitting lid. A 1-pint canning jar works well. Heat the soy sauce and 1/3 cup water in a small saucepan over medium heat just to a boil. Pour the liquid over the mushrooms. Let stand until cool. Cover the jar and steep the mushrooms in the soy mixture in a cool place until the soy sauce develops a pronounced mushroom flavor, 2 to 3 weeks.

2. Strain the liquid through a fine sieve into clean jar with a tight fitting lid. Press on the mushrooms to extract as much liquid as possible. Store in the refrigerator for up to 6 months.