Begin by lining a 9x9-inch baking dish with parchment paper, leaving some overhang for easy removal later.
In a double boiler, combine the bittersweet chocolate and butter. Stir occasionally until smooth and fully melted.
Once your chocolate and butter mixture is melted, remove it from the heat. Add peanut butter and stir until fully combined and smooth.
Pour in sweetened condensed milk and vanilla extract. Stir well until everything is thoroughly combined.
Carefully pour the fudge mixture into your prepared baking dish, using a spatula to spread it evenly.
Before the fudge sets, sprinkle a pinch of sea salt over the top.
Refrigerate the fudge for at least 2 hours, or until it is firm to the touch.
Once set, use the parchment overhang to lift the fudge out of the baking dish. Cut it into squares or rectangles.
