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

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

Add 15 ounces of marshmallows to the pot and stir gently, allowing the marshmallows to melt into a gooey blend, about 3-5 minutes.

Remove the pot from heat and stir in 1 teaspoon of vanilla extract.

While still warm, fold in 6 cups of Rice Krispies cereal until each piece is evenly coated.

Transfer the mixture to the prepared pan and press firmly and evenly into the pan.

Cool at room temperature for about 30 minutes, then slice into bars.