In a large bowl, cream butter and brown sugar until light and fluffy. Beat in the egg, milk and vanilla. Beat in chocolate until blended. Combine the flour, baking soda and salt; gradually add to creamed mixture and mix well. Stir in walnuts.
Drop by tablespoonfuls 2 inches apart onto greased baking sheets. Bake at 350 degrees for 8-10 minutes or until set. Cool on pans for 1 minute before removing to wire racks to cool completely.
In a large bowl, beat frosting ingredients until smooth. Frost cookies.