Add butter, cocoa, sugar and evaporated milk to a saucepan.
Cook over medium heat until it reaches a boil.
Lower flame to low and simmer 5 minutes.
Remove from heat; stir in vanilla.
Use an immersion blender to thoroughly incorporate the cocoa or carefully pour the hot mixture into a blender and blend for a minute.
Serve while warm, at room temperature, or chilled.
Store leftovers in a lidded container in the refrigerator. If it it too thick to pour after being refrigerated, microwave it for 15 - 30 seconds.