PREP: Line pan with parchment paper, leaving an overhang for easy removal (or line with foil). Do not skip this step or you probably won't be able to get the fudge out looking very nice. Cut the butter into 1-tablespoon-sized pieces.
COMBINE: In a thick-bottomed nonstick pan, heat the butter, chocolate chips, salt, and sweetened condensed milk on the lowest heat setting. Using a heat-resistant spatula, stir the mixture constantly making sure to scrape the sides and bottom. Resist the urge to increase the heat-- your patience will pay off!
ADD FLAVOR: As soon as the butter, condensed milk, and chocolate chips are melted, remove the pan from heat. Stir in the vanilla extract; mix well.
CHILL: Transfer the mixture to the prepared pan. Spray a spatula with nonstick cooking spray and smooth the fudge into one even layer with a smooth, even top. Cover and refrigerate for 3-5 hours or until fudge is firm.
SERVE: Remove from the pan using the overhang of parchment paper. Cut the fudge with a hot, very sharp knife. (Run it under hot water, dry, and then make quick cuts.) I recommend cutting the fudge into small pieces -- it's rich!
STORAGE: Fudge is best stored in an airtight container in the fridge and enjoyed cold. If it's left out of the fridge for long, the fudge becomes dry. Leftover fudge will stay fresh 1-2 weeks in an airtight container in the fridge, but we like it best when used within 2-3 days.
FREEZING: Start by cutting the fudge into squares and wrapping each piece individually in plastic wrap (as airtight as possible). Then place each wrapped piece in a large freezer bag. Seal, removing as much air as possible. If you don't wrap each piece individually, be sure to separate pieces with wax paper so they don't freeze and stick together. Thaw pieces in the fridge.
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Notes
Note 1: Pan size: For thicker fudge, use an 8x8-inch pan; photos in the post show the thickness of the fudge using a 9x9-inch pan.Note 2: Chocolate: High-quality chocolate makes such a difference in the taste of this Fudge Recipe. My favorite brands are Ghirardelli® and Guittard® (not sponsored). Use whatever flavor of chocolate you like best or a combination of a couple of kinds. We like milk and dark chocolate mixed (half and half) or plain milk chocolate best!