1. Place butter in medium microwave-safe bowl or 4-cup glass measure. Microwave at HIGH (100%) 1 to 1-1/2 minutes or until melted. Add cocoa; stir until smooth. Add sugar, salt and evaporated milk; stir until well blended.
2. Microwave at HIGH 5 to 8 minutes, stirring occasionally, or until mixture comes to a boil.
3. Beat with whisk until smooth. Stir in vanilla. Serve warm sauce over cake, ice cream or other desserts. Cover; refrigerate leftover sauce. About 3 cups sauce.
For gift-giving, pour sauce into heat-proof jars. Cover; refrigerate. Decorate with ribbon, bow and recipe for sauce, if desired.
To reheat: Microwave at HIGH a few seconds at a time until warm. (Time will depend on amount and temperature of sauce.)
CHOCOLATE-NUT SAUCE: At the end of cooking, stir in 1 cup coarsely chopped toasted nuts.
CHOCOLATE-MINT SAUCE: Stir in 1/2 teaspoon peppermint extract with vanilla extract.