1. Preheat oven to 350 degree F. Lightly grease a 13x9x2-inch baking pan; set aside. Prepare brownie batter according to package directions. If desired, stir in the 1/2 cup peanuts. Spread batter in prepared pan. In a medium mixing bowl, beat together cream cheese, 1/2 cup peanut butter, the milk, and egg yolk until smooth. Spoon mixture into a decorating bag fitted with a small round tip (or spoon it into a resealable plastic bag and snip off one corner of the bag). Pipe mixture over brownie batter in pan.
2. Bake in preheated oven for 30 minutes. Cool in pan on a wire rack for 1 hour.
3. In a medium mixing bowl, stir together the frosting and remaining 1/2 cup peanut butter. Spread over cooled brownies; if desired, sprinkle with additional peanuts. Cut into small squares. Makes 64 bites.