Preheat oven to 350F. In large bowl, combine flour, brown sugar,and egg; cut in cold butter until crumbly. Stir in peanuts. Reserve 2 cups crumb mixture. Press remaining mixture on bottom of 13x9 inch baking pan.
Bake 15 minutes or until lightly browned. Meanwhile, in another large mixing bowl, beat sweetened condensed milk, peanut butter and vanilla. Spread over prepared crust; top with reserved crumb mixture.
Bake an additional 25 minutes or until lightly browned. Cool. Cut into bars. Store covered at room temperature.