In heavy skillet over medium heat, cook bacon until crispy.
Remove all but 2 Tbsp. of the bacon grease.
Turn the heat to low and add in the onions and salt then cook until browned or lightly caramelized. Stir in the garlic and cook for an additional 1 minute.
Remove all but 1/2 Tbsp. of the grease.
Add in brown sugar, cider vinegar, black pepper, cayenne pepper, cumin, and prepared black coffee.
Let cook down until slightly thickened and lightly browned, or about 20-25 minutes.
Remove from heat and stir in balsamic vinegar.
Serve warm or in the refrigerator in an airtight container or mason jar.