Cream the butter, shortening, and sugar until well-mixed.
Add the eggs one at a time, mixing well after each addition.
Mix in the vanilla. If desired, add 1/2 tsp. of salt.
In another bowl, sift the 3 cups of flour with the 1/2 tsp. of baking powder
Alternately add the dry mixture and milk until all is combined.
Stir in walnuts.
Pour batter into 2 greased 8 or 9-inch cake pans.
Bake at 350 degrees for 50-60 minutes or until done. (see note on temperature below)
Remove when the cake is done and cool.
Frost as desired.