Preheat oven to 325 degrees F.
Grease and flour a 10-inch tube or Bundt pan; set aside.
Mix together all the dry ingredients (flour, baking powder and salt) and set aside.
In a mixing bowl or stand mixer, beat butter, shortening, and sugar until light, creamy, and fluffy. Add the flavoring and continue beating.
Add eggs, one at a time and beating well after each addition.
Add the flour mixture alternately with the milk, stirring until blended and smooth. Stir in the black walnuts.
Spoon batter into prepared pan and place pan on a rimmed baking sheet (especially if using a removable bottom tube pan). Bake at 325 for 1 hour 25 minutes (check a little earlier -- mine took the full 85 minutes) or until cake tester inserted in middle comes out clean.
Cool cake in pan for 20 minutes. Invert cake onto a wire rack to cool completely, about 2 hours. Dust with confectioners’ sugar before serving if desired.