Add butter, cocoa powder, sugar and evaporated milk to a medium saucepan and stir to combine. Bring to a boil over medium heat. Boil for seven minutes.

Remove from heat. Stir in vanilla. Use an immersion blender and blend the sauce in the pan for 2 - 4 minutes. Take care not to splatter the hot sauce and burn yourself. Serve hot. Store any leftovers in the fridge, and reheat in the microwave to serve.