Preheat oven to 375 degrees.

In a small bowl, stir together the flour, salt, and baking soda; set aside. 

Using an electric mixer, cream together margarine, the ¾ cup granulated sugar, and brown sugar for 2 to 3 minutes or until light and fluffy. Add the egg and beat just until combined. Add the vanilla and the peanut butter and beat until combined.

Add the flour mixture a little at a time until well combined, scraping the sides of the bowl as needed. Chill the dough for 30 minutes before shaping.  

Shape rounded teaspoonfuls of dough into balls and roll in sugar. Place 2 inches apart onto an ungreased cookie sheet or a cookie sheet lined with parchment paper. 

Bake for 9 to 11 minutes until edges are set. Cookies may look a little underdone. Allow cooling on the cookie sheet for 3 to 4 minutes before transferring to a wire rack to cool.