Preheat oven to 350 degrees F. Sift all-purpose flour with baking powder and salt. Stir in whole-wheat flour. In a large bowl, combine egg, vanilla, sugar, and butter. Beat until well mixed. Stir in milk. Add flour mixture and combine until blended. Do not overbeat. Add nuts if desired. Pour into greased 9x5-inch loaf pan and bake 1 hour. Cool in pan 10 minutes before removing.