In a large mixing bowl, cream butter and sugar until light and fluffy. Beat in peanut butter, egg and vanilla. Thoroughly blend flour and baking soda. Gradually add to creamed mixture. Fold in chocolate chips. Refrigerate dough for 1 hour. Preheat oven to350 F. Shape dough into 2 cm balls. Place on ungreased cookie sheets, 8 cm apart. With the bottom of a glass, press each ball into a cookie 4 cm in diameter. Bake for 15 to 20 minutes or until lightly browned. From : the Ultimate Peanut Butter Cookbook by Arthur Boze