Prepare the baking dish: Line a square baking dish with parchment paper. This ensures easy removal of the fudge once it has set and allows for clean cutting.
Melt the chocolate mixture: In a medium saucepan over medium heat, combine 2 cups white chocolate chips, 1 can (14 oz) sweetened condensed milk, and 1/2 cup unsalted butter, cubed. Stir continuously until the mixture is melted and smooth.
Add additional flavors: Once the mixture is smooth, remove the saucepan from the heat. Stir in 1 teaspoon vanilla extract, a pinch of salt, and 3/4 cup of the chopped pistachios. Mix until evenly distributed.
Spread the mixture: Pour the fudge mixture into the prepared baking dish, spreading it evenly with a spatula. Ensure it reaches the corners for even thickness.
Add finishing touches: Sprinkle the remaining 1/4 cup pistachios on top of the fudge, gently pressing them into the surface to secure them as it sets.
Chill and set: Place the baking dish in the refrigerator and chill for at least 3 hours, or until the fudge is firm to the touch.
Cut and enjoy: Once set, carefully remove the fudge from the baking dish using the parchment paper. Cut into squares and enjoy your homemade pistachio fudge!