Pour the Coke, sugar, lemon juice, cinnamon, and optional spice into a wide skillet or saucepan (more surface area = faster). Bring to a gentle boil over medium heat.
Reduce heat to maintain a steady simmer and let it bubble for 20–40 minutes, stirring now and then. You want the liquid to reduce by about half.
Dip in a cold spoon. If the syrup just coats the back and leaves a clear line when you swipe your finger through it, it’s ready. Don’t wait for it to look thick while hot, it will thicken a lot when cold.
Remove any whole spices, stir in the vanilla extract, and let it cool completely. If you accidentally take it too far and it’s sticky or taffy-like once chilled, don’t panic, just return it to the pan, add a splash of water, reheat gently, and loosen it back to syrup consistency.
