Peanut butter and chocolate meet (just as they should) in these rich bar cookies.
In a medium mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add granulated and brown sugars. Beat until thoroughly combined.
Add egg, the 1/3 cup peanut butter, the baking soda, vanilla, and salt. Beat until thoroughly combined. Stir in flour and oats. Spread into a greased 13x9x2-inch baking pan.
Bake in a 350° oven for 15 to 18 minutes or until edges are light brown.
Sprinkle with chocolate pieces; let stand for 5 minutes to soften.
Meanwhile, for icing, stir together powdered sugar, the 3 tablespoons peanut butter and enough of the milk, a tablespoon at a time, to reach drizzling consistency.
Spread softened chocolate evenly over bars. Drizzle with icing. Cool in the pan on a wire rack before cutting into bars.