Preheat the oven to 350F. Butter two 9” round cake pans and place parchment round in the bottom of each. Butter the parchment. Set aside.
Add flour, sugar, baking powder and soda, and salt to the bowl of a stand mixer. Mix on low speed until combined, about 30 seconds.
On low speed, add the butter. Increase the speed to medium and continue mixing until you reach a sandy consistency.
Turn off the mixer and add the remaining ingredients. Beat on medium-high speed until pale yellow and fluffy, about 1-2 minutes. Be careful not to overmix.
Divide the batter between the two prepared pans and smooth the top with a spatula.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Cool the cakes in the pan for 15-20 minutes. Run a knife around the edge of the cakes to loosen and turn them out onto racks. Remove the parchment paper and let cool completely. Using a serrated knife, remove the dome from the cakes.
For the frosting, sift the powdered sugar and cocoa powder into a small bowl. Add the butter and salt to the bowl of a stand mixer fitted with the paddle attachment and cream until light and fluffy, about 3-4 minutes. Add half of the powdered sugar mix and beat on low speed to incorporate. Increase the speed to high and beat in the remaining ingredients until smooth. If the frosting is too stiff, thin with more buttermilk one tablespoon at a time.
Place one cake on a large plate and spread a thick layer of frosting on top. Place the other cake on top and continue to spread the frosting on the top and sides, smoothing with an offset spatula.