Prepare the Pan: Line a 9x13-inch baking pan with foil or parchment paper, leaving some overhang. Lightly spray with nonstick cooking spray and set aside.

Melt the Butter: In a large pot over medium-low heat, melt 4 tablespoons of unsalted butter. Stir until fully melted.

Melt the Marshmallows: Add 15 ounces of mini marshmallows to the pot. Stir continuously until melted, about 3-5 minutes.

Combine with Pudding and Vanilla: Stir in the pistachio pudding mix and vanilla extract until smooth, about 1-2 minutes.

Add the Rice Krispies: Fold in 6 cups of Rice Krispies cereal into the marshmallow mixture until evenly coated.

Press into the Pan: Using wax paper or greased hands, press the mixture into the prepared pan evenly.

Cool and Slice: Let cool at room temperature for about 30 minutes, then lift from the pan and slice into bars.