Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, letting it hang over the sides like handles. Lightly grease the pan with nonstick spray or a little butter first so the parchment sticks and stays in place.
1 piece parchment paper, nonstick spray or butter
Roughly chop the dried cranberries and pistachios so you have a mix of chunks and smaller bits. Set them aside in separate small bowls.
3/4 cup dried cranberries, 3/4 cup pistachios
In a medium heavy-bottomed saucepan, add the white chocolate chips, sweetened condensed milk, and butter. Place over low heat; white chocolate scorches easily, so keep the heat low and steady.
3 cups white chocolate chips, 14 oz sweetened condensed milk, 4 tablespoons unsalted butter
Stir constantly with a silicone spatula or wooden spoon, scraping the bottom and sides of the pan as the mixture melts. Cook on low until the mixture is completely melted, smooth, glossy, and thick, about 4–7 minutes depending on your stove.
3 cups white chocolate chips, 14 oz sweetened condensed milk, 4 tablespoons unsalted butter
Remove the pan from the heat. Stir in the vanilla extract, fine sea salt, and orange zest if using. Taste carefully (mixture will be hot) and add a tiny extra pinch of salt if needed to balance the sweetness.
1 teaspoon pure vanilla extract, 1/2 teaspoon fine sea salt, 1 teaspoon orange zest
Let the mixture sit off the heat for 2–3 minutes until very warm but not scorching. Gently fold in the chopped dried cranberries and pistachios until evenly distributed, being careful not to over-stir as the fudge will start to thicken.
3/4 cup dried cranberries, 3/4 cup pistachios
Immediately scrape the fudge mixture into the prepared pan. Use a spatula to spread it evenly, nudging it into the corners and smoothing the top. If desired, sprinkle a few extra cranberries and pistachios on top and gently press them in so they stick.
Let the fudge cool at room temperature for 20–30 minutes, then transfer the pan to the refrigerator. Chill for at least 2 hours, or until the fudge is firm enough to slice cleanly. For best texture, chill overnight.
Use the parchment overhang to lift the set fudge out of the pan and onto a cutting board. With a sharp knife, cut into 1- to 1 1/2-inch squares. For neat edges, warm the knife under hot water, dry it, and wipe it clean between cuts.
Arrange fudge squares on a serving tray or pack them in parchment- or wax-paper-lined tins and boxes. Place small pieces of parchment or wax paper between layers to prevent sticking.