Melt condensed milk and chocolate in microwave (using microwave safe bowl) for about a minute and a half or until the milk is hot and watery and the chocolate chips are beginning to melt. DO NOT OVERHEAT (will burn chocolate and leave a clumpy/grainy mixture with a bad taste).
Take bowl out and stir until mixture is smooth and well blended.
Add peanut butter for peanut butter/chocolate fudge OR add salt and vanilla. Or add all three, depending on your tastes.
Stir until all is well mixed, smooth, even and shiny.
Pour mixture into an UNGREASED foil lined cake/brownie pan.
Put in fridge until mixture is hard (overnight is best).
Take out of fridge and cut into squares (or other desired shapes) and put into a Tupperware or other covered/airtight container.