In a large bowl, combine the flour, brown sugar and 1/2 cup butter. Press into an ungreased 13-in. x 9-in. baking pan. Bake at 350° for 12-15 minutes or until lightly browned.
Sprinkle with marshmallows and return to the oven for 3-5 minutes or until marshmallows begin to melt; set aside.
In a large saucepan, cook and stir the peanut butter chips, corn syrup, vanilla and remaining butter until smooth. Remove from the heat; stir in cereal and peanuts. Pour over prepared crust, spreading to cover. Cool on a wire rack before cutting into bars.