Line an 8x8 inch baking pan with parchment paper and grease lightly.
Chop pistachios and measure all ingredients.
Combine chocolate chips and condensed milk in microwave-safe bowl.
Microwave in 30-second intervals, stirring until smooth.
Stir in butter, vanilla, and optional food coloring.
Fold in chopped pistachios, reserving some for topping.
Pour mixture into prepared pan, spread evenly.
Sprinkle pistachios on top, press gently.
Refrigerate for 2 hours or until firm.
Cut into squares with sharp knife, clean between cuts.