Preheat oven to 180g degrees C. Grease and flour a 23cm 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 centre 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 icing sugar, 1 tablespoon of butter, and 1 tablespoon of port wine until smooth and spreadable. Spread over the cooled bars.