Preheat oven to 325°F (160°C).

In a bowl, cream softened butter with an electric mixer until light and fluffy.

Gradually add sugar and brown sugar, beating until well combined and fluffy.

Add eggs one at a time, mixing on low speed until just blended after each addition.

In a separate bowl, stir together buttermilk and baking soda.

In another bowl, combine flour and salt.

Alternately add the flour mixture and buttermilk mixture to the butter mixture, beginning and ending with the flour mixture, mixing on low speed until just combined after each addition.

Stir in vanilla extract.

Spray five 5 3/4 x 3 1/4 inch disposable foil loaf pans with cooking spray.

Pour batter evenly into the prepared pans (about 1 cup batter per pan).

Bake in the preheated oven for 50-55 minutes, or until a pick inserted into the center comes out clean.

Cool cakes in pans on wire racks for 10 minutes.

Remove cakes from pans and cool completely on wire racks.