Preheat oven to 180 degrees C. Grease one 20cm square pan.
Cream the butter or margarine, add the sugar and continue creaming until light and fluffy. Beat in the eggs one at a time. Stir in the maple essence.
Combine the flour, baking powder and salt. Add the flour mixture in 3 parts alternately with the milk to the butter mixture, beginning and ending with flour. Stir in the walnuts and pour in the prepared pan.
Bake for 40 to 45 minutes or until an toothpick inserted in the centre comes out clean. Let cake cool then ice with maple butter icing.