Beat butter at medium speed with an electric mixer until creamy. Gradually add sugars, beating until light and fluffy. Add eggs, 1 at a time, beating at low speed just until blended after each addition.
Stir together buttermilk and baking soda. Combine flour and salt; add to butter mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Beat at low speed after each addition just until blended. Stir in vanilla.
Spray 5 (5 3/4- x 3 1/4-inch) disposable foil loaf pans with baking spray. Pour batter evenly into pans (about 1 cup batter in each pan).
Bake at 325° for 50 to 55 minutes or until a wooden pick inserted in center comes out clean. Cool cakes in pans on wire racks 10 minutes. Remove from pans, and cool completely on wire racks.