In a large bowl, cream together the butter, brown sugar and granulated sugar.
In the same bowl, blend into the butter and sugar mixture the egg, peanut butter, salt, baking soda, and vanilla.
Into the wet ingredients, add the flour and oatmeal and mix until there is no more visible flour.
Lightly grease a 9x13 baking dish before spreading the cookie mixture into the pan.
Bake at 350 for 20-25 minutes or until the edges are golden brown and a toothpick poked in the center comes out clean.
Evenly sprinkle 1 cup of chocolate chips over the pan of cookie bars. Once it is melted, spread the chocolate over the surface.
Let the cookies cool 10 minutes while you prepare the peqnut butter icing. In a small bowl, mix together with a whisk the powdered sugar, 1/4 cup of peanut butter and milk. Pour the mixture over the top of the chocolate covered cookie bars and use a spatula to gently swirl the icing on top of the chocolate.
Eat right away if you can't wait, or let them cool completely for easy cutting!