Preheat oven to 375F. Sift the granulated and brown sugars together. Beat the butter until soft. Add the sifted sugars gradually to the butter until creamy. Beat in the egg, peanut butter, salt and baking soda. Sift the flour before measuring; then add the flour to the batter along with the vanilla. Roll the dough into small balls. Place them on a greased cookie sheet. Press them flat with a fork. Bake for 15 minutes. These cookies are rich and crumbly and they ttaste great.