My stepdaughter makes these spontaneously, making the house smell so wonderful!
You don't need to roll in sugar and criss cross with a fork, but it's a nice touch.
Preheat oven to 375 degrees. Combine margarine, peanut butter, sugar, egg, and vanilla. Cream, then blend in dry ingredients. Form into balls and roll in granulated sugar, place on ungreased cookie sheet. With a floured fork, criss cross each ball to flatten. Bake for 10 minutes, cool on wire rack.