Lightly brush kombu clean without removing its coating of white dust. Place in a large saucepan with 6 cups of cold water. Slowly bring to a simmer over low heat and cook for 5 minutes. Remove the kombu and discard it. Add 1 cup cold water and the sprouts to the broth. Return to simmer for 5 minutes. Increase heat to high. Once the stock boils, remove from the heat. Strain the stock through a cheesecloth-lined sieve, discarding the sprouts. Stock may be stored in refrigerator for up to 3 days or frozen for up to 6 months. Makes 5 cups.