Combine all of the ingredients in a heavy-bottom saucepan over medium-low heat. Bring to a simmer, reduce the heat to low, and gently simmer until dark and thickened, about 30 minutes.
Test the thickness of the sauce by dipping a wooden spoon and raising it slowly. The sauce should drip smoothly from the spoon. If the sauce has reached the desired thickness, then remove pan from heat and let cool to room temperature. The sauce can be stored in a sealed jar for up to 2 weeks.