Combine the butter, sugar, cocoa powder and evaporated milk in a large saucepan and bring to a boil. Boil for 7 minutes while whisking frequently.
Remove from heat and stir in the vanilla.
Transfer to blender and blend for about 2 minutes, (make sure to remove the center piece of the blender top and cover with a towel any time you blend hot liquids.)
Serve immediately or let sit until ready to use. Store in the refrigerator and reheat as needed.