Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving some overhang on two sides to create handles. Lightly grease under the parchment if needed to help it stay in place.

In a medium heavy-bottomed saucepan, add the white chocolate chips, sweetened condensed milk, and unsalted butter.

Place the saucepan over low heat. Stir constantly with a heat-safe spatula or wooden spoon until the white chocolate is almost fully melted and the mixture is thick, glossy, and nearly smooth. Remove from the heat and continue stirring until completely smooth.

Off the heat, stir in the vanilla extract, fine sea salt, and orange zest (if using) until evenly distributed and fragrant.

Gently fold in the chopped dried cranberries and chopped pistachios, reserving a small handful of each if you want to sprinkle some on top. Work fairly quickly as the mixture will start to thicken.

Scrape the fudge mixture into the prepared pan. Use a spatula to spread it evenly into the corners and smooth the top. Sprinkle reserved cranberries and pistachios on top if desired, gently pressing them into the surface. Add a pinch of flaky sea salt, if using.

Cover the pan loosely with plastic wrap or foil and refrigerate for 2–3 hours, or until the fudge is firm but not rock-hard. It should give just slightly when pressed.

Use the parchment handles to lift the chilled fudge out of the pan onto a cutting board. With a long, sharp knife, cut into about 1-inch squares, wiping the knife clean between cuts for neat edges. Serve on a platter or place in small paper candy cups for gifting.