1 1/2   c            Sifted flour
     1/4   ts           Baking powder
     1/4   ts           Baking soda
     3/8   ts           Salt
   1                    Egg
   2                    Egg whites
     1/2   c            Low-fat buttermilk
   1 1/4   ts           Vanilla
     1/4   c            Stick margarine
   1       tb           Unsalted butter
   1       c            Sugar
  1. Have all ingredients at room temperature (70F).
  Adjust oven racks to divide oven in thirds; preheat
  oven to 325F. Spray 5 cup loaf pan, bundt pan or tube
  pan with nonstick cooking spray. 2. In small bowl,
  whisk together flour, baking powder, baking soda, and
  salt; sift together and set aside. 3. In small bowl,
  whisk together egg and egg whites; set aside. 4. In
  small bowl, combine buttermilk and vanilla; set aside.
  5. Cut margarine and butter into 1/4" chunks and place
  in large bowl of electric mixer; beat at medium speed
  until softened, about 1 minute. Add sugar gradually,
  beating constantly, until smooth, about 3 minutes.
  Gradually dribble in beaten eggs, beating at
  medium-high speed for 2-3 minutes. On low speed, beat
  in 1/3 of the reserved flour mixture, scraping sides
  of bowl. On medium-high speed, gradually dribble in
  1/2 of the buttermilk mixture, scraping sides of bowl.
  On low speed, beat in half of the remaining flour
  mixture. On medium-high speed, beat in the rest of the
  buttermilk mixture, scarping sides of bowl. On low
  speed, beat in the remaining flour mixture until well
  combined (batter may look curdled). Pour into prepared
  pan and bake until cake starts to pull away from the
  sides of the pan, the top is golden brown and a
  toothpick inserted in the center of the cake comes out
  clean, 35-40 minutes for bundt or tube pan, or 65-70
  minutes for loaf pan. 6. Cool on rack 10-15 minutes;
  invert pan and unmold onto rack to cool completely.
  Cut into 12 slices.