1. Line an 8-inch or 9-inch square baking dish with non-stick foil or parchment paper for easy removal after setting.
2. Prepare your tools: use a heavy-based pan for stovetop cooking or a large microwave-safe bowl for the microwave method.
3. Combine 3 cups of white chocolate chips and a 14-ounce can of sweetened condensed milk in your cooking vessel.
4. If using the microwave method, heat at 70% power for 90 seconds, then stir until smooth. If stovetop, melt the mixture over low heat while stirring constantly to avoid sticking or burning.
5. Add a 3.4-ounce package of instant pistachio pudding mix to the melted base and stir thoroughly until fully incorporated, ensuring it dissolves completely for a smooth texture and vibrant green color.
6. Roughly chop 1 ½ cups of salted pistachios and fold them into the fudge mixture, distributing them evenly for a consistent crunch. Substitute with other nuts, marshmallows, or dried fruits if desired.
7. Pour the fudge mixture into the prepared baking dish, spreading it evenly with a spatula and smoothing the top for a clean finish. Sprinkle additional crushed pistachios on top if you like.
8. Let the fudge set in the refrigerator for 2-3 hours or at room temperature for 4-5 hours until firm.
9. Once set, lift the fudge out of the dish using the foil or parchment lining.
10. Use a sharp knife, dipped in hot water and wiped dry between slices, to cut the fudge into squares for smooth, clean edges.
11. Store the fudge in an airtight container at room temperature for up to two weeks or in the refrigerator for longer freshness. For extended storage, freeze the fudge for up to three months and thaw before serving.