In a large bowl, combine the margarine, brown sugar and white sugar. Beat the mixture until it is smooth and fluffy. Stir in the eggs and vanilla. Beat again, making a smooth, satiny mixture. Add the flour, baking powder and salt. Mix with a big spoon until the batter is thick and smooth. Stir in the peanuts, if using. Turn the batter into a well oiled 9-13-inch rectangular pan. Bake at 350F for 25 to 30 minutes. The bars will rise up and then fall slightly. Remove fom the oven when done. Allow the bars to cool before cutting into 24 or 32 pieces. If desired, you may frost these with chocolate or vanilla frosting. These are a pleasant change from brownies.
To fluff up your flour before measuring, just stir it with a spoon to incorporate some air. If you can't sift or stir your flour, then reduce the amount called for to about 2 1/2 cups.