Preheat oven to 350 degrees. Line an 8x8x2-inch square baking pan with parchment paper so paper hangs over all four sides.
In a large bowl, combine flour, brown sugar and butter. Stir, using wooden spoon, until large clumps form and then use hands to knead mixture together. Press the dough into the bottom of the prepared pan. Bake for 12 minutes. Remove the pan from the oven and place on cooling rack to cool completely.
In medium saucepan, combine sweetened condensed milk and peanut butter chips. Cook over medium heat, stirring occasionally, until chips are melted and mixture is smooth. Add marshmallows and stir well. Cook and stir until the marshmallows melt and into the peanut butter mixture. Add 1 cup peanuts; stir to incorporate. Pour the mixture over crust and spread evenly using spatula.
Sprinkle remaining 1 1/4 cup peanuts on top of filling while still warm. Gently press peanuts into the filling using your fingertips. Refrigerate bars 1 hour or more before cutting.
Store bars at room temperature in an airtight container or covered in plastic wrap.