Preheat your oven to 350 degrees Fahrenheit. This is a crucial step as it ensures even cooking right from the start.
Prepare the baking pan by lining an 8-by-8-inch baking pan with aluminum foil, leaving an overhang. This will help you lift the bars out later. Don’t forget to spray it with cooking spray to prevent sticking!
Make the crust. In a medium bowl, combine 6 tablespoons of softened unsalted butter, ¾ cup all-purpose flour, and ¼ cup light brown sugar. Stir until the mixture is sandy and crumbly. It should feel a bit like wet sand.
Transfer the mixture into the prepared pan. Press it down firmly with your fingertips, then pack it down using the bottom of a glass. This will form a sturdy crust that holds everything together.
Bake the crust in your preheated oven for 10 to 12 minutes. Keep an eye on it; it should be set and pale in color. Don't worry if it seems slightly under-baked; it will go back in the oven!
Prepare the filling by evenly sprinkling 3 cups of miniature marshmallows over the baked crust. Return the pan to the oven and bake for about 5 minutes, just until the marshmallows start to melt. Watch closely to ensure they don’t brown.
Make the topping. In a large microwave-safe bowl, combine 1 heaping cup of creamy peanut butter, ⅓ cup light corn syrup, and ¼ cup unsalted butter. Heat in the microwave for about one minute to melt everything together, then stir well.
Add the flavor. Mix in 1 ½ teaspoons vanilla extract, followed by 1 ½ cups of peanuts and 1 ½ cups Rice Krispies. Stir until everything is fully combined.
Pour this delicious mixture over the melted marshmallows, smoothing it down with a spatula to create an even layer. This step ensures every bite is packed with goodness!
Cool the bars. Set the pan aside to cool for at least 4 hours before slicing. If you're in a hurry, refrigerating the bars can expedite the cooling process!
Slice the bars by lifting them out of the pan using the foil overhang. Transfer to a cutting board and cut them into squares. Store in an airtight container at room temperature for up to one week, or freeze for up to three months.