Line a 15x10x1-inch baking pan with foil, extending the foil over edges of pan; set aside.
For crust: In a large bowl, combine oats, the 1 cup flour, the 1-1/2 cups brown sugar, the 1 teaspoon baking powder and the 1/2 teaspoon baking soda. Using a pastry blender, cut in the 1 cup butter until mixture resembles coarse crumbs. Remove 1 cup of crumb mixture to reserve for topping. Stir peanuts and peanut butter-flavored pieces into the 1 cup reserved topping mixture; set aside.
Press remaining crumb mixture onto the bottom of prepared baking pan. Bake in a 350 degree F oven for 10 minutes. Cool at least 15 minutes in pan on a wire rack.
In a medium saucepan, heat the 1 cup brown sugar, the 1/4 cup butter and peanut butter over medium heat until butter melts and mixture is smooth, stirring constantly. Cool slightly. Stir in egg and vanilla. Stir in the 1-1/4 cups flour, the 1/2 teaspoon baking powder and the 1/4 teaspoon baking soda. Drop in very small mounds evenly over the crust in pan. Sprinkle with reserved topping mixture.
Bake in a 350 degree F oven for 18 to 20 minutes or until a wooden toothpick inserted near center comes out clean. Cool completely on wire rack before lifting from pan using edges of foil and cutting into bars.
For icing: In a small bowl, stir together powdered sugar and enough of the milk to make an icing of drizzling consistency. Drizzle icing over bars. Makes 32 bars.
