Make a quick double boiler (stovetop method): Add an inch of water to a small pot and bring it to a gentle simmer. Place a heatproof bowl on top, making sure the bottom doesn’t touch the water.

Melt the base: To the bowl, add the butter and white chocolate chips. Stir often until mostly melted and smooth.

Add the condensed milk: Pour in the sweetened condensed milk and keep stirring over low heat until everything is silky and well combined.

Finish with peanut butter: Remove the bowl from heat. Add the peanut butter (and vanilla and salt if using). Stir until the mixture is completely smooth and glossy.

Pour and level: Scrape the mixture into the prepared pan. Smooth the top with a spatula. Sprinkle chopped peanuts on top if you want a little crunch.

Chill to set: Refrigerate for 2–3 hours, or until firm enough to slice cleanly.

Cut and serve: Use the parchment handles to lift the fudge onto a cutting board. Warm a sharp knife under hot water, dry it, then cut into 1–1.5-inch squares.