Preheat the oven to 350 degrees F. Prepare three 9-inch round baking pans by lining the bottoms with parchment paper (trace the pan and cut out the circle) and grease the sides. Set the pans aside.

In a medium bowl, whisk the flour, cocoa powder, baking powder, baking soda, and kosher salt together. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars together on medium speed for 3-4 minutes. Add the eggs, one at a time, beating for a minute or so after adding each egg. Add the vanilla extract.

Lower the speed of the mixer to low, add half of the flour mixture, and beat until it is well-incorporated. Slowly stream in the buttermilk until it is all combined. Add the remaining flour mixture and beat until it is just combined. Finish stirring everything with a spatula, if necessary.

Divide the batter between the 3 pans. I recommend using a small kitchen scale to weigh the pans to ensure even amounts of batter in each.

Bake the cakes for 20-25 minutes, or until a toothpick or knife comes out clean when inserted into the center. Rotate the pans about halfway through to ensure even baking.

Let the cakes cool in the pans for about 20 minutes before turning them out onto wire racks to finish cooling completely.

You can double-wrap the cakes in plastic wrap or foil and store them in the freezer if you aren't going to frost them immediately. 

Add the softened butter to the bowl of a stand mixer fitted with the paddle attachment. Beat it on medium for about 1 minute.

While the butter is being beaten, whisk together the cocoa powder and powdered sugar.

Add 2 cups of the powdered sugar/cocoa powder mixture, the kosher salt, and vanilla extract. Beat on low speed until the powdered sugar is incorporated.

Add another 2 cups of the powdered sugar. Beat on low speed, and add an additional 2 cups of powdered sugar as needed to thicken the frosting. Use the milk/heavy cream to thin things out as needed and the remaining cups of powdered sugar to thicken if needed.

Once the frosting is as stiff as you’d like it to be, beat it on medium speed for about 4 minutes, or until the frosting is fluffy and smooth.

Frost the completely cooled cakes however you'd like. I went with rosettes all over.