Beat butter at medium speed with an electric mixer until creamy.
Add sugar gradually, mixing well.
Add eggs and mix until well blended.
Combine flour, baking powder, soda, and salt in a bowl.
Add alternately with buttermilk, beginning and ending with the flour mixture. Add vanilla.
Pour batter into a greased & floured 13" x 9" pan.
Cover and refrigerate overnight.
Preheat oven to 350°.
Let cake stand at room temperature for 30 minutes.