Melt the chocolate and butter over *low* heat. Add the sugar and stir well. Slowly add the milk, stirring constantly. Heat over low heat while stirring occasionally until thickened. Add the vanilla, pour over ice cream and enjoy. Or eat it plain.
It's hard to mess this up. When my aunt makes it she puts all the ingredients in a pan and heats it till they mix together and it tastes just as good. My mom is a purist and only adds almost exactly four oz of evaporated milk. So her sauce takes a little less time to thicken and is a little thicker over all. My uncle adds all 5oz of milk and his sauce is great too. You can store extra in the fridge and heat it in the microwave, or just grab a spoonfull and eat plain! THis stuff is really yummy on Ben and Jerry's coffee heath toffe crunch ice cream!