Preheat the oven to 350ºF. Grease and flour two 9-inch baking pans, ideally using cocoa powder (see note), and line the bottoms with parchment circles. In the large bowl of a stand mixer, combine the flour, sugar, cocoa powder, baking soda, and salt, mixing on low speed for one minute. Add the eggs, buttermilk, melted butter, and vanilla extract, then beat on medium speed until the batter forms, about 30 seconds. Pause to scrape down the sides and bottom of the mixer bowl as well as the paddle. Beat for an additional 15 seconds until the mixture is mostly smooth.