Combine margarine, peanut butter, sugars, egg, and vanilla. Blend together very well.
Add flour, baking soda and salt with mixer. Chill dough 15 minutes in the refrigerator. Roll into balls by hand.
Place on sprayed cookie sheet and bake in a preheated 350 degrees F (175 degrees C) oven until edges very lightly browning (about 7 to 10 minutes). Don't overcook. We store them in a plastic storage containers, and they stay soft!