Remove the label from an unopened can of sweetened condensed milk
Place the unopened can on its side in a large pot
Cover the can completely with water, keeping it submerged at all times
Bring the water to a gentle simmer
Simmer for 2 to 3 hours for a light caramel sauce or 3 to 4 hours for a darker, thicker caramel
Check the water level every 20 to 30 minutes and add hot water as needed
Turn off the heat and let the can cool completely before opening
Open the can carefully
Stir the contents until smooth
Use immediately or transfer to a jar and refrigerate
For a stovetop version, pour sweetened condensed milk into a saucepan
Add butter and brown sugar if desired
Cook over low heat, stirring constantly, until thickened and caramel-colored
Remove from heat and stir in vanilla and a pinch of salt if desired
Let cool before using