1/4 cup soy sauce
1/4 cup + 1/8 cup (i.e one half of a quarter cup) brown sugar
In small pot, add brown sugar and soy sauce.
Boil the sauce over low to medium flame until thickens resembling maple syrup. If the mixture starts to boil vigorously and looks like it is going to overboil, leave the pot away from the flame until the boiling has calm down and continue to boil over low heat. You have to keep a watchful eye while the mixture is cooking to avoid overboiling. As the mixture cools down, it will further thicken.