1. 1. Have all ingredients at room temperature (70F).

2. Adjust oven racks to divide oven in thirds; preheat oven to 325F.

3. Spray 5 cup loaf pan, bundt pan or tube pan with nonstick cooking spray.

4. 2. In small bowl, whisk together flour, baking powder, baking soda, and salt; sift together and set aside.

5. 3. In small bowl, whisk together egg and egg whites; set aside.

6. 4. In small bowl, combine buttermilk and vanilla; set aside.

7. 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.

8. Add sugar gradually, beating constantly, until smooth, about 3 minutes.

9. Gradually dribble in beaten eggs, beating at medium-high speed for 2-3 minutes.

10. On low speed, beat in 1/3 of the reserved flour mixture, scraping sides of bowl.

11. On medium-high speed, gradually dribble in 1/2 of the buttermilk mixture, scraping sides of bowl.

12. On low speed, beat in half of the remaining flour mixture.

13. On medium-high speed, beat in the rest of the buttermilk mixture, scarping sides of bowl.

14. On low speed, beat in the remaining flour mixture until well combined (batter may look curdled).

15. 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.

16. Cut into 12 slices.