Preheat your oven to 350 degrees. Spray a 9×13 metal pan on the bottom, line with parchment paper, and spray again. If you need to use metal binder clips to hold down the sides, remember to use metal, not plastic. Set aside
In a stand sized mixer fitted with the paddle attachment, combine the flour, sugar, cocoa powder, baking soda, baking powder, and salt. Mix on low until evenly combined.
In a small bowl, whisk the eggs, buttermilk, hot water, vanilla extract, and vegetable oil until evenly combined.
With the mixer on the lowest speed, pour the liquid ingredients into the dry ingredients and mix just until combined. There should be no flour streaks. With a rubber spatula, scrape the bottom of the bowl to make sure the batter is evenly combined.
Evenly distribute the cake batter to the prepped cake pan. With the back of a spoon, even out the batter to ensure it’s smooth for baking.
Bake for 40-45 minutes. Check the cake by inserting a toothpick. It’s done when it comes out with just a few moist crumbs on it.
Let it completely cool in the cake pan. While the cake cools, make the chocolate buttercream.
For the fudgy buttercream
In a microwave safe bowl, heat the chocolate for 30 seconds, stir, heat for 30 more seconds, and set aside. The chocolate will not be melted all the way, but this will help it melt quicker when you add the cream.
In another microwave proof bowl, heat the cream until nearly boiling. Make sure it doesn’t boil over. Pour the hot cream over the slightly warm chocolate chips. Let it sit for 2 minutes and then stir. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter for about 2 minutes on medium-high speed.
Add the chocolate/cream mixture. Scrape down the sides and bottom of the bowl with a rubber spatula and beat for 1 more minute.
Add the powdered sugar, cocoa powder, vanilla, and salt. Mix on low until combined, then turn the mixer to medium high and beat for 3 minutes.
With a wooden spoon or rubber spatula, beat by hand to mix out any air pockets.
When the cake is cooled, use an offset spatula and frost the top of the cake. This cake is good served at room temperature or cold. However you serve it, it will not disappoint!