Combine brown sugar and soy sauce in sauce pan and bring to boil. Boil for about 3 minutes, stirring constantly, and let it cool a bit (10 minutes) to see if it's thickening up. If it's not sticky, slowly add more brown sugar, a little at a time, and bring back to boil, cooking about 5 more minutes. Remember it will thicken up as it cools. If it's too thick, add some mirin (or sherry cooking wine or white wine), just a little at a time, and stir.