Line an 8×8-inch pan with parchment paper, leaving overhang for easy removal.
In a large microwave-safe bowl, combine white chocolate chips, condensed milk, and butter.
Microwave in 30-second intervals, stirring well after each, until fully melted and smooth.
Stir in pistachio pudding mix, vanilla extract, and salt until thick and well combined.
Add food coloring if using, mixing just until the color is even.
Fold in chopped pistachios, reserving some for topping if desired.
Pour mixture into the prepared pan and smooth the top.
Sprinkle reserved pistachios on top and gently press them in.
Refrigerate for 1–2 hours until fully set.
Lift out and slice into squares with a sharp knife.
Notes
• Use instant pistachio pudding only; cook-and-serve will not set properly.
• Stir well between microwave intervals to avoid scorching.
• Do not overheat—stop as soon as the mixture is smooth.
• Chill completely before slicing for clean, neat squares.