In a medium saucepan, heat dark brown sugar, heavy cream, butter, and dark corn syrup over medium heat. Bring to a boil. It will get very bubbly.
Lower heat and simmer gently for 10 minutes, stirring every 1-2 minutes. Do NOT stir constantly. If needed, lift pan off burner if it simmers too hard.
After 10 minutes, spoon a bit of sauce on a plate. Press spoon against sauce and scrape to make sure there is no graininess.
Remove from heat and stir in bourbon. It will bubble and foam. Keep stirring and let alcohol burn off.
Stir in vanilla and a large pinch of salt. Taste-test. Add more salt if desired.
Allow to cool slightly before serving.
Notes
• It goes against every instinct to just let sugar simmer without stirring. Despite this, resist!• Store in refrigerator up to 1 week.