Cream butter and peanut butter together. Add sugar gradually and cream thoroughly. Add well beaten egg to creamed mixture, Sift flour once before measuring. Sift flour, soda, baking powder and salt together and add to creamed mixture. Chill dough well; then form into balls the size of a walnut. Place balls on lightly greased baking sheet. Flatten with fork dipped in flour, making criss - cross pattern. Bake.