Preheat oven to 350 degrees F. Line an 8 inch square pan with foil and spray with baking spray or grease well and dust with flour.
Mix together the butter, peanut butter, granulated sugar, 3 tablespoons of the brown sugar, vanilla and egg yolk. Add flour and stir just until mixed. Stir in the peanuts. Spread batter across bottom of prepared pan using wet fingers.
Sprinkle the chocolate chips and marshmallows evenly over the batter and press lightly.
Beat the egg white until stiff and beat in the brown sugar. Spread egg white mixture over marshmallow mixture (I stopped about 1/4 inch from the edges of pan leaving edges uncovered) and bake at 350 for 25 minutes or until meringue is brown.
Cool in pan set on a wire rack for a few hours. Lift foil from pan and cut into squares. At this point, I recommend chilling the bars/squares for a while. They're easier to cut when cold, and should you decided to cut them into even smaller squares, it will be easier.
Makes 16 squares (of which some will be crusty edges).