Line an 8x8-inch baking pan with parchment paper, leaving some overhang on two sides to create handles. Lightly mist with nonstick spray or rub with a tiny bit of butter to make removal and slicing easier.

In a medium, heavy-bottomed saucepan, combine the sweetened condensed milk and white chocolate chips. Set over low heat and stir constantly with a heat-safe spatula until the chips are almost fully melted and the mixture is smooth and glossy. Keep the heat low to avoid scorching the white chocolate.

Add the creamy peanut butter to the warm mixture. Continue stirring over low heat until everything is fully combined and the mixture is thick, smooth, and shiny, like a soft peanut butter frosting.

Remove the pan from the heat. Stir in the vanilla extract and fine sea salt until evenly distributed. Taste a tiny bit carefully and adjust the salt to your liking.

Immediately scrape the warm fudge mixture into the prepared pan. Use an offset spatula or the back of a spoon to spread it into an even layer, pressing it into the corners. Work fairly quickly, as the fudge begins to set as it cools.

While the surface is still soft, sprinkle flaky sea salt, chopped peanuts, or festive sprinkles over the top, gently pressing them in so they adhere.

Transfer the pan to the refrigerator and chill for at least 2 hours, or until the fudge is firm enough to slice cleanly. For the neatest squares, chill 3–4 hours or overnight.

Use the parchment handles to lift the fudge from the pan onto a cutting board. Run a sharp knife under hot water, dry it, then cut the fudge into about 1-inch squares, reheating and wiping the knife between cuts for clean edges.