Heat oven to 375°F. (For dark or nonstick pans, heat oven to 350°F.) In large bowl, beat 1 cup softened butter, seeds from 1 vanilla bean, and 1 1/2 cups sugar with electric mixer on high speed until light and fluffy. Add eggs, one at a time, beating well after each addition. Stir in 1 teaspoon vanilla. On low speed, beat flour mixture into sugar mixture alternately with 1 cup milk, beating after each addition just until smooth. Scrape side of bowl occasionally. Stir in sour cream.