PREP: Line an 8x8-inch pan with parchment paper, leaving an overhang for easy removal (or line with foil). Do not skip this step, or the fudge won't remove nicely. Cut the butter into 1-tablespoon-sized pieces.
PEANUT BUTTER BASE: In a thick-bottomed, nonstick pan, heat 2 tablespoons of the butter, Reese's peanut butter baking chips, half of the sweetened condensed milk (198g -- use a food scale if you have one!), and 1/8 teaspoon fine sea salt on low. 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 mixture is melted, remove the pan from heat. Stir in 1 teaspoon of the vanilla extract; mix well by folding the extract into the fudge with the spatula. Pour into the prepared pan and use the spatula to smooth and press the fudge into one even layer. Place the pan in the fridge while preparing the milk chocolate top layer.
MILK CHOCOLATE TOPPING: Quickly clean and thoroughly dry the pan. Heat the remaining 2 tablespoons of the butter, milk chocolate chips, the remaining half of sweetened condensed milk, and the remaining 1/8 teaspoon fine sea salt on low. Using a heat-resistant spatula, stir the mixture constantly, making sure to scrape the sides and bottom. Once melted, remove from heat and fold in the remaining 1 teaspoon vanilla extract. Mix until well combined. Remove the other fudge from the fridge and pour the milk chocolate layer on top. Use the spatula to gently coax and spread the fudge to fully cover the bottom layer.
CHILL: Cover and refrigerate for 4-8 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: We love Reese's Fudge straight out of the fridge; it's so delicious chilled! Make sure to store the fudge in an airtight container; cold air will dry out the fudge quickly.
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Notes
Note1: We love milk chocolate chips in this fudge, but for a less-sweet fudge, use semi-sweet or dark chocolate chips instead. Use good-quality chocolate chips for the best flavor.