Stir together flour, baking soda, and slat. In a mixer bowl, beat butter for 30 seconds. Add peanut butter and sugars, beating until fluffy. Add egg and vanilla, beating well. Add dry ingredients to beaten mixture, and blend thoroughly. Shape dough into 1-inch balls, and roll in granulated sugar if desired. Place on ungreased cookie sheet and criss-cross by pressing with tines of a fork. Bake at 375 for about 10 minutes. Cool about 1 minute before removing to wire rack.