Begin by lining a 9x13-inch baking pan with aluminum foil or parchment paper. Spray with nonstick cooking spray.

In a large pot over medium-low heat, add ½ cup of unsalted butter and 1 cup of creamy peanut butter. Stir gently until smooth, about 4 minutes.

Stir in 15 ounces of marshmallows and mix until fully melted, about 3-5 minutes, until thick and gooey.

Remove the pot from heat and quickly stir in 1 teaspoon of vanilla extract, making sure to mix thoroughly.

Gradually fold in 6 cups of Rice Krispies until well coated, using a spatula to avoid crushing the cereal.

Transfer the mixture into your prepared pan, spreading it evenly and pressing down firmly to create a compact layer.

Allow the treats to cool at room temperature for about 30 minutes, then lift out of the pan and slice into bars.