Preheat the oven to 350° F / 176°C. Prepare three 8-inch cake pans by lining the bottoms with parchment paper and spraying the bottom and sides liberally with non-stick spray. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, combine the unsalted butter and granulated sugar. Beat together on low speed until the butter and sugar are combined. Scrape down the sides of the bowl. Increase the speed to medium high and beat for another 2 to 3 minutes or until the mixture is light in color and fluffy.
Add the vanilla extract and one of the eggs to the mixer and beat until the egg is fully combined. Scrape down the sides of the bowl. Add the remaining eggs one at a time, beating after each egg until fully incorporated. Turn the speed up to medium high and beat for 1 minute until the mixture is fully emulsified and fluffy.
Meanwhile, combine the all-purpose flour, baking powder, baking soda, and kosher salt in a mixing bowl. Whisk together until there are no lumps and set aside.
Combine the buttermilk and vegetable oil in a measuring cup and whisk together.
Add one third of the flour mixture to the mixer and beat on low just until the flour is mostly incorporated. Add half of the buttermilk mixture and beat just until it is mostly incorporated. Repeat with another third of the flour and the rest of the buttermilk. Add the last of the flour mixture and beat on low just until the batter is fully combined.
Scrape down the sides and bottom of the bowl making sure there are no streaks of unmixed butter in the batter. Divide the batter between the three prepared cake pans, spreading evenly. Bake in the preheated oven for 27 to 30 minutes or until done.
When the cakes are done, leave in the pans for 5 minutes before turning out onto a wire rack to cool completely.