4. With the mixer at low speed, add a third of the flour mixture, then half of the buttermilk, and mix. Add another third of the flour and mix. Mix in the remaining buttermilk and vanilla extract. Mix in remaining flour until smooth.
5. Pour the batter into the pan. Bake at 350 until set in the center and springy, 25 to 35 minutes. Let cool a little, then run a serrated knife around the edge of the pan. Turn out the cake, upside down, onto a serving platter.