Line an 8×8-inch square baking dish with parchment paper, leaving some overhang on the sides.

In a medium saucepan, combine white chocolate chips and sweetened condensed milk. Heat over low to medium heat, stirring frequently until melted and smooth.

Once melted, remove from heat and stir in cocoa powder and vanilla extract until fully incorporated.

Gently fold in the chopped pistachios until evenly distributed.

Transfer the fudge mixture to the prepared baking dish and spread evenly with a spatula.

Refrigerate for at least 2 hours or until set.

Once set, lift the fudge from the pan using parchment overhang, cut into squares and serve.