Combine chocolate, sweetened condensed milk and salt in a microwave safe bowl. Heat together for 1 minute, stir and repeat as necessary to melt chocolate (doesn't take more than 75-90 seconds total in my microwave).
Remove from microwave and stir to ensure all chocolate is melted. Add vanilla and your choice of mix in - combine thoroughly.
If making the peanut butter variation, smooth fudge in pan, dollop peanut butter on top by teaspoonfuls and swirl through fudge with chopstick or knife.
Pour into a foil-lined 8" square pan and chill for 1 hour. If you chill it more than that, it may be harder to cut.
Cut into 1" squares and either serve or wrap in foil or waxed paper squares for gifts.