Preheat oven to 350ºF. Butter and line two 8-inch round baking pans with parchment paper. Butter parchment.In a large mixing bowl, combine the sugar, flour, cocoa, baking soda, baking powder and salt, whisking until thoroughly combined. Add eggs, buttermilk, water, oil, and vanilla and beat with an electric mixer on medium speed until all dry ingredients are incorporated. Divide evenly among prepared pans.Bake for 40 to 45 minutes (30 to 35 minutes for a 6-inch cake) or until a toothpick inserted in the center comes out with just a few moist crumbs attached. Remove from oven and place on a wire rack until cool enough to handle. Run a small knife around the edges of each pan, then gently invert onto wire racks. Cakes should come out cleanly. Peel off parchment and allow cake layers to cool completely. At this point, the cakes can be frozen until ready to use, up to 1 month. Simply wrap each layer individually in a double layer of plastic wrap and then store inside a large zip top bag (I find frozen cakes to be much easier to work with later, and always freeze mine even if I’m finishing the cake the next day).To prepare frosting, in the bowl of a stand mixer fitted with the paddle attachment, beat butter on medium speed until very pale and creamy, about 5 minutes. Add sifted powdered sugar and cocoa powder and mix to combine. Add sour cream, hot water, vanilla and salt and mix on low speed until the liquid is incorporated, scraping down the sides of the bowl as needed. It may look slightly curdled at this point, but don’t worry. Increase speed to medium-high and beat until smooth, another 2-3 minutes. Add lukewarm chocolate and mix until smooth and glossy. At this point you have about 30 minutes at cool room temperature (longer in the summer) to work with the frosting before it begins to harden (this frosting doesn’t do particularly well when made ahead of time).To assemble, level your cake layers first if necessary (if there is any sort of domed top, you want to trim that off with a serrated knife). Place one layer, bottom side up, on a cake stand or serving platter (on a piece of parchment if you will need to move it). Spread about 1 cup of frosting in an even layer, then place second layer on top, flat side up, pressing gently to adhere.Cover the entire cake with a thin layer of buttercream using an offset spatula. This “crumb coat” will seal in the cake crumbs and make the final layer of frosting easier. If your cakes were frozen this should firm up pretty quickly, otherwise refrigerate for about 10 minutes to set.Slather with remaining buttercream in a thick layer, reserving some for piping decorative details if desired.