In a large mixing bowl, combine the melted butter and mini marshmallows. Microwave for 30 seconds, stir, and repeat until the marshmallows are completely melted and smooth.

Add the chopped pistachios to the melted marshmallow mixture and stir until well combined.

Gradually fold in the Rice Krispies until evenly coated with the marshmallow mixture.

Press the mixture firmly into a greased 9x9 inch baking pan.

In a microwave-safe bowl, melt the dark chocolate chips in 30-second intervals, stirring in between until smooth.

Pour the melted chocolate over the Rice Krispie base and spread evenly using a spatula.

Allow to cool at room temperature for about 10 minutes, then refrigerate for 30 minutes to set the chocolate.

Once set, cut into squares and serve.