Combine sugar, cocoa, and salt in a large saucepan. Slowly stir in the evaporated milk. Bring to a boil over medium heat, stirring constantly. Cook and stir for 6-8 minutes, or until thickened.
Remove from heat and stir in butter and vanilla. Stir till butter melts. Cool slightly before pouring into glass jars.
Serve warm, and store leftovers in the refrigerator. It heats up nicely in the microwave.
Notes
I like my chocolate sauce rich, so I use the full amount of cocoa powder. You get a more milk chocolate flavored sauce if you use less cocoa.
When I am making this just for my family, I cut the recipe in half and only boil it for 3-4 minutes. (Half a can of evaporated milk is about 3/4 cup.)