Preheat oven to 350 degrees; grease a 13 x 9 x 2-inch baking pan. In a medium mixing bowl, beat the butter with an electric mixer for 30 seconds. Add the sugars; beat until thoroughly combined. Add the egg, 1/3 cup peanut butter, baking soda, vanilla extract and salt. Beat until thoroughly combined. Stir in the flour and oats. Spread in the prepared pan. Bake for 15 to 18 minutes, or until the edges are light brown. Sprinkle with the chocolate pieces and let stand for 5 minutes to soften. Meanwhile, stir together the powdered sugar, 1/4 cup peanut butter and enough of the milk, a tablespoon at a time, to drizzle. Spread the chocolate evenly over the top and then drizzle with the peanut butter mixture. Cool in the pan on a wire rack. Cut into bars.