In a large pot, melt the butter over medium heat. Once melted, add all but 1 cup of the mini marshmallows and continue stirring until the mixture is smooth and fully melted.
Remove the pot from heat and stir in the dry pistachio pudding mix and vanilla extract until fully incorporated.
Fold in the rice krispies cereal and the remaining 1 cup of mini marshmallows using a spatula. Mix until everything is well combined.
Pour the mixture into a 9-inch square baking dish lined with parchment paper. Use the spatula to press the mixture evenly into the pan.
Let the treats cool and set for at least one hour. Once set, remove from the dish and peel away the parchment paper. Cut into 16 squares.
Drizzle the tops of the squares with melted dark chocolate and allow the chocolate to set for about 15 minutes.
Enjoy immediately, or store in the refrigerator for a cool and refreshing treat!