Preheat oven to 350 degrees F. Line a 9-inch square pan with greased foil or parchment paper with overhanging sides.

With an electric mixer, beat the butter and brown sugar on high until smooth and creamy, about 2 minutes. Scrape down the bowl and add the egg and vanilla. Add the flour, baking powder, baking soda, and salt and mix on low until combined. Press evenly into the bottom of the prepared pan.

Bake until edges are golden and center is set, 15-18 minutes. Let cool completely.

Add the marshmallow cream and peanut butter to a microwave-safe bowl and microwave on high 30-60 seconds until smooth and pourable. Stir in the powdered sugar. Spread evenly on top of the blondie base. Sprinkle the peanuts over the top.

In a medium saucepan over medium low heat melt caramels with heavy cream until smooth. Alternatively, place both caramels and heavy cream in a microwave-safe bowl and microwave on high for 60 seconds. Stir. Continue to microwave in 30 second intervals, stirring after each, until smooth.

Immediately poor caramel over the top of the bars, using a spatula if needed to spread it evenly. Chill in the fridge for about 30 minutes until filling and topping are set.

Use overhanging foil or parchment to lift bars out the pan, and slice as desired. Bars are best stored in the fridge.