Combine first 3 ingredients, and set aside. Beat margarine and peanut butter at medium speed of a mixer until light and fluffy. Gradually add sugars, beating at medium speed until well-blended. Add vanilla and egg white, and beat well. Add flour mixture; stir well. Turn dough out onto wax paper; shape into a 6-inch log. Wrap log in wax paper; freeze 3 hours.
Preheat oven to 350°.
Cut log into 24 (1/4-inch) slices, and place slices 1 inch apart on a baking sheet coated with cooking spray. Bake at 350° for 8 to 10 minutes. Remove from pan, and cool on wire racks.
Chocolate-Peanut Butter Variation: Add 1 ounce of grated semisweet chocolate to the flour mixture.