Preheat the oven to 325°F / 165°C. Generously grease an 8 by 4-inch / 20 by 10cm loaf pan, then line it with greased parchment paper. (****see Baker's Notes)
In a medium bowl, whisk together both flours and the salt, then set aside.
In a stand mixer fitted with the paddle attachment, cream the butter on medium-high speed until creamy and smooth, about 1 minute.
Turn the mixer speed to low, add the sugar to the butter, and mix until incorporated. Turn the speed to medium-high and beat until very light and fluffy, about 5 minutes. Scrape the bowl often for even incorporation.
Add the vanilla extract and vanilla seeds, turn the speed to medium, and mix until incorporated.
Turn the speed to medium-low and add the eggs, one at a time, beating just until combined. Scrape the bowl after each addition.
Add one-third of the flour mixture to the butter mixture, mixing on low speed, just until combined. Add half the buttermilk and mix on low speed just until combined. Repeat with another one-third flour, the remaining buttermilk, and the final one-third flour, scraping the bowl and paddle after each addition.
Pour the batter into the prepared pan. Smooth the top and tap the pan on the counter a few times to release excess air bubbles. Set the pan on a baking sheet.
Bake until a tester comes out clean, about 50 minutes (It can take up to 1 hour 10 minutes depending on the shape of the pan you use). Let the cake cool in the pan for 10 minutes, then remove from the pan and place onto a serving plate and let cool completely before serving.