Prepare the Frosting:
In a pot, melt the butter and salt over medium heat. Once melted, whisk in the heavy cream and sugar, continuing to whisk occasionally until the mixture is nearly boiling (about 7-8 minutes). Stir in the chocolate chips until they are completely melted and the mixture is smooth.
Add the sifted Dutch-processed and black cocoa powders to the bowl of a stand mixer. Slowly stream the hot chocolate mixture into the bowl while mixing until fully combined. Scrape down the sides of the bowl and mix for 2-3 minutes until the frosting is smooth and lump-free. Stir in the vanilla extract.
Pour the frosting into a container and refrigerate for a minimum of 4-6 hours, or preferably overnight, before using.
Bake the Cake Layers:
Preheat your oven to 350°F (180°C). Line three 6-inch cake pans with rounds of parchment paper on the bottoms.
In a medium bowl, whisk together the flour, baking soda, and salt until well combined. Set aside.
In a large bowl, whisk together the oil and sugar until well mixed. Add the buttermilk, eggs, and vanilla extract, whisking until smooth. Gradually whisk in the dry ingredients until the batter is fully combined.
In a separate bowl, whisk together the hot coffee and black cocoa powder until smooth and lump-free. Then, whisk this mixture into the cake batter until just combined.
Evenly distribute the batter into the prepared cake pans. For accuracy, use a food scale; each pan should hold approximately 395 grams of batter.
Bake the cakes for 25-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow the cakes to cool in the pans on a wire rack. Once cooled, run an offset spatula around the edges and flip them out onto the rack.
Assemble the Cake:
When ready to assemble, allow the frosting to come to room temperature for about 1 hour. Using a stand mixer with a paddle attachment, mix the frosting for 30-60 seconds to loosen it. Be careful not to overmix, as it will start to lighten in color.
Place the first cake layer on your serving board. Pipe an even layer of frosting on top, then place the next cake layer on top of the frosting. Continue this process, placing the last cake layer upside down for a flat top. Chill the assembled cake in the fridge for 30-60 minutes, or until the frosting is firm.
Use the remaining frosting to cover the top and sides of the cake. For added texture, use the back of a spoon to create swoops in the frosting. Decorate the cake as desired, then serve and enjoy!