Prepare a 9×13-inch pan by spraying it with butter-flavored cooking spray. Set aside.
In a large pot over low heat, melt the butter.
Add 14 ounces of the mini marshmallows and stir until mostly melted and smooth. Then add the brown sugar.
Remove the pot from the heat and stir in the peanut butter, vanilla extract, and salt until fully combined and creamy.
Fold in the Rice Krispies cereal until evenly coated.
Let the mixture cool for about 1 minute, then fold in the remaining 3 ounces of mini marshmallows to create extra-gooey marshmallow pockets throughout the treats.
Transfer the mixture to the prepared pan and gently press into an even layer. Avoid packing the treats down too firmly. Let it sit for 30 minutes.
Place the chocolate chips in a medium heat-safe bowl.
Pour the heavy cream into a microwave-safe measuring cup or bowl. Microwave for 45 to 60 seconds, or until the cream is hot and steaming around the edges. Do not allow it to boil.
Immediately pour the hot cream over the chocolate chips. Let the mixture sit undisturbed for 2 minutes.
Whisk until smooth and glossy. If any chocolate remains unmelted, microwave the mixture in 10-second intervals, stirring after each, until completely smooth.
Pour the chocolate ganache over the rice krispie treats and allow to sit at room temperature for an hour or refrigerate for 30 minutes or until the chocolate has set.
Let cool completely before slicing into squares.