It’s important to have everything ready once you turn on your heat, so measure out our chocolate, butter, and vanilla in a bowl and set aside. Spray an 8×8 baking pan with non-stick spray.
Add sugar, marshmallows, and evaporated milk to a large sauce pan. Turn the heat under the pan to medium high and start stirring. Keep stirring until the mixture starts to boil.
Once you reach a boil, continue to cook and stir for 8 minutes. Whatever you do, don’t stop stirring. When 8 minutes is up, remove the pot from the heat, but don’t stop stirring.
Either grow a third arm or get a helper to carefully add the chocolate, butter, and vanilla to the boiling sugar mixture. Be careful. You do NOT want to get this molten mixture on your skin. Keep stirring until everything is melted and blended.
Pour the mixture immediately into the waiting baking pan and refrigerate for several hours or overnight to set.
Cut into small pieces and try really hard not to eat every last bit.
My cryptic family recipe calls for one can (5.3 ounces) evaporated milk. My grocery store only had 12 ounces cans. I don’t know if they make this size anymore, but you can always just weigh it with your kitchen scale or use a touch more than 5/8 of a cup.