Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9 inch square pan.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the egg, then stir in the vanilla, milk and 2 tablespoons Port wine. Combine the flour, chopped walnuts and baking powder; stir into the creamed mixture. Spread evenly into the prepared pan.
Bake in the preheated oven for 15 to 20 minutes, until a knife inserted into the center comes out clean. Brush the top with 2 tablespoons of the Port wine as soon as it comes out of the oven. Let cool to room temperature before glazing.
To make the glaze, beat together the confectioners' sugar, 1 tablespoon of butter, and 1 tablespoon of port wine until smooth and spreadable. Spread over the cooled bars.