Lightly grease your 9x13-inch baking dish with cooking spray or line it with parchment paper for easy removal later.
In a large saucepan over low heat, melt the unsalted butter. Once melted, add the mini marshmallows and stir continuously until smooth.
Stir in the pistachio pudding mix and vanilla extract until well combined, noticing the mixture's lovely green hue.
Remove from heat and add the Rice Krispies cereal, stirring gently until fully coated. Avoid crushing the cereal.
Pour the mixture into the prepared baking dish. Press down evenly with a buttered spatula or greased hands.
Allow to cool at room temperature for about 30 minutes. Cut into squares or rectangles and serve.
