Preheat the oven to 350 degrees F. Grease and flour 2 (8 1/2 by 4 1/2 by 2 1/2-inch) loaf pans. Line the bottoms with parchment paper.
Cream the butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment for about 5 minutes, or until light and fluffy. With the mixer on medium speed, beat in the eggs, 1 at a time.
In a large bowl, sift together the flour, baking powder, baking soda, and salt. In another bowl, combine the buttermilk and vanilla. Add the flour and buttermilk mixtures alternately to the batter, beginning and ending with the flour. Divide the batter evenly between the pans, smooth the tops, and bake for 45 minutes to 1 hour, until a cake tester comes out clean.
When the cakes are done, let them cool for 10 minutes. Take them out of the pans, place them on a baking rack and allow them to cool completely. Wrap well, and store in the refrigerator.