In large bowl with electrix mixer at high speed, beat butter, granulated sugar, eggs, and vanilla, occasionally scraping side of bowl with scraper, until light and fluffy- about 3 minutes.
In small bowl, combine banana and buttermilk.
At low speed, beat in flour mixture (in fourths) alternately with banana mixture in 3rds- about 1 minute.
Stir in dates and nuts.
Bake 1 hour and 10 or 15 minutes.
Cool on rack 15 minutes in pan, then turn out on rack until cool.