Heat the sugar: Add the brown sugar to a medium saucepan and heat over medium-low heat.
Melt the sugar into syrup: Stir with a wooden spoon until the brown sugar melts and becomes a dark syrup. Note: Decrease the heat if the liquid begins to smoke before it turns dark.
Add boiling water: Once the color changes from amber to black, remove it from the stove and slowly add the boiling water, a little at a time. Be aware that the sauce will splatter at this point, so protect your hands.
Let it rest, store or use: Allow the sauce to cool to room temperature, and pour into an airtight container. Use as needed or store in the refrigerator. The sauce will last for up to 6 months in the fridge.
