In another bowl, cream margarine, butter, salt and sugar till fluffy. Add eggs in one at a time, only after the previous egg has been thoroughly mixed into the batter.
Add vanilla and sifted ingredients into the batter and mix well. Pour into a greased and lined 8 inch square pan and bake for 50 minutes.
Note: it might start getting kinda brown at 20 mins, so I strongly reccomend that you tent it with tin foil after around 20 mins, to prevent the top from burning due to the large amt of butter in the recipe.