Sift together flour, salt, baking powder and baking soda. Cream butter with sugar, brown sugar and peanut butter, creaming after each addition until light and fluffy. Add egg and mix thoroughly. Add flour mixture gradually. Beat until smooth. Drop mixture, 1 teaspoon at a time and about 2″ apart on to an undressed baking sheet. Flatten with a fork 2 times to form a criscross pattern on the cookie surface. Bake in a 350 F oven for 8-10 min. Remove from oven and cool.