Preheat the oven to 325°F. Coat a 10-inch bundt pan thoroughly with baking spray.
Cream the butter and sugar in a stand mixer fitted with the paddle (or with a hand mixer) until smooth and pale.
Beat in the eggs one at a time, mixing well after each addition.
Mix in the sour cream and vanilla just until combined.
Add the cake flour on low speed and mix only until no dry streaks remain.
Remove the bowl from the mixer. Stir in the buttermilk by hand, scraping the sides of the bowl.
Spoon the batter into the prepared pan and smooth the top. Bake 75 to 80 minutes, checking at 1 hour. The cake is ready when a toothpick inserted in the center shows a few crumbs and the edges are deep golden.
Cool the cake in the pan until it is fully set, then turn it out onto a rack and let it cool completely.