Prepare the pan: Line an 8x8-inch baking pan with parchment paper, letting the edges overhang for easy lifting. Lightly mist with nonstick spray if your parchment slides.

Melt the chocolate and butter: In a microwave-safe bowl, combine chocolate chips and butter. Microwave in 20–30 second bursts, stirring between each, until mostly melted. Alternatively, melt in a heatproof bowl set over a pot of simmering water (double boiler), stirring until smooth.

Stir in the condensed milk: Pour in the sweetened condensed milk and stir until fully combined and glossy. If needed, warm for another 10–15 seconds to loosen.

Add peanut butter, vanilla, and salt: Stir in the peanut butter, vanilla, and salt until the mixture is thick, smooth, and uniform. Work fairly quickly to keep everything silky.

Add mix-ins (optional): Fold in chopped peanuts or mini chips for texture. If using jam, hold it for the swirl in the next step.

Spread and smooth: Scrape the fudge into the prepared pan. Use an offset spatula or the back of a spoon to smooth the top. If swirling jam, dollop small spoonfuls over the surface and lightly drag a knife through to create streaks. Finish with a pinch of flaky sea salt if you like.

Chill to set: Refrigerate for 2–3 hours, or until firm enough to cut. For faster setting, place in the freezer for 45–60 minutes, then transfer to the fridge.

Slice and serve: Lift the fudge out using the parchment overhang. Warm a sharp knife under hot water, wipe dry, and cut into 1–1.5-inch squares. Serve slightly chilled or at cool room temperature.