1Heat oven to 350^. Lightly spray large cookie sheets with cooking spray or line with parchment paper.
2In a large bowl, beat brownie mix, peanut butter, the eggs and vanilla with electric mixer on low speed for 1/2 minute. Beat on high for another 1/2 minute or until mixed well. Stir in chocolate chips.
3Drop 36 heaping tablespoons of dough 2 inches apart on the cookie sheets. Press a slight indention in center of each cookie. Bake 9 to 11 minutes or until cookies are set when gently touched. Cool on cookie sheet for 1 minute; remove from cookie sheets to cooling rack. Cool completely, about 20 minutes.
In a small bowl, beat frosting, peanut butter and the butter with an electric mixer on medium speed until smooth. Fill each thumbprint indentation with 2 teaspoons frosting mixture, spreading slightly; sprinkle with peanuts. Let stand until frosting mixture is set. Store loosely covered in single layer with wax paper in between.