*Not having port on hand in our test kitchen, we substituted a combination of 4 1/2 teaspoons red wine (Cabernet Sauvignon) and 1 1/2 teaspoons brandy for each 2 tablespoons port, which seemed to work well.
Preheat the oven to 350°F. In a medium-sized bowl, cream together the butter and sugar until light and fluffy. Add the egg and vanilla, and beat well. Stir in the milk and 2 tablespoons port.
In a separate bowl, combine the nuts, flour, baking powder and salt. Fold the nut mixture into the butter mixture, mixing only until all ingredients are incorporated. Spread the batter into a lightly greased 9 x 9-inch pan.
Bake for 20 to 25 minutes, or until a cake tester inserted into the center comes out clean. Transfer the pan to a cooling rack and immediately brush the bars with the remaining 2 tablespoons port. Cool to room temperature.
When the bars are cool, mix the butter, salt, sugar and port together, using additional liquid if necessary to achieve a spreadable consistency. Cover the bars evenly with the glaze, sprinkle with the ground walnuts, and let stand until the glaze is firm.