Prepare a 9×13-inch baking pan by lining it with aluminum foil or parchment paper. Spray with nonstick cooking spray.

In a large pot, melt ½ cup of unsalted butter over medium-low heat. Add in 1 cup of creamy peanut butter and stir until smooth.

Stir in 15 ounces of marshmallows into the melted peanut butter mixture and stir until completely melted and smooth.

Remove from heat and stir in 1 teaspoon of vanilla extract until fully incorporated.

Gently fold in 6 cups of Rice Krispies cereal until evenly coated without crushing the cereal.

Transfer the mixture into the prepared baking pan and press down with a greased spatula until smooth and level.

Allow to cool at room temperature for about 30 minutes or until set, then slice into bars or squares.