In a microwavable bowl, combine the contents of one bag of semisweet chocolate chips (about 11 oz, give or take an ounce), one can of sweetened condensed milk (it's a half height can, it only comes in the one size around here), and one square (one ounce) unsweetened chocolate. Microwave one minute. Stir. Repeat until smooth and even in consistency, and the unsweetened chocolate has melted and blended in... about 5 minutes total. Stir in one teaspoon vanilla extract. Spread into an 8x8" greased pan, about 1" deep. Chill until firm, about an hour. Cut into 1" squares.
I warn you, it's some of the most luscious, rich fudge you'll ever have. Anyone you give some to will want more. It's excellent for bribing coworkers to get stuff done on time. ("If you can get me that by the end of the day, I'll make a pan of homemade fudge for you...")
To make fudge with a taste of mint in the fudgyness, use a mint flavor extract instead of the vanilla extract.
To make peanut butter fudge, make the recipe as described above, but do not stir in any flavor extract. Instead, stir in one cup chunky peanut butter. I recommend using an organic brand with strong natural peanut flavor.