Preheat your oven to 350°F/175°C. Butter or grease three 8-inch baking pans and line it with parchment paper. If you only have two pans, bake the first two cake layers and let them cool slightly before reusing one of the pans for the third layer.
In a stand mixer fitted with the paddle attachment, combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In a large jug or bowl, whisk together the buttermilk, oil, eggs, and vanilla extract.
While the mixer is on your lowest speed, slowly pour in the wet ingredients into the dry.
While the mixer is still running, slowly add the hot water and mix. Your batter will be very thin, so don't worry! Make sure the mixer is on your lowest speed so it doesn't splash everywhere!
Pour the batter into the prepared pans, it's about 470 grams per cake pan. Bake for 22-25 minutes, until a toothpick comes out clean when inserted
Allow the cakes to cool for 10 minutes before removing from the pans. Let them cool completely on a cooling rack.
Make Filling
In a small saucepan, bring the heavy cream to a simmer (but not a boil!).
Place the chopped chocolate in a large bowl. Pour the hot heavy cream on top and let sit for a few minutes. Whisk after 2-3 minutes until smooth. Let sit until completely cool.
You'll know you're ready for the next step when you can easily scoop out the ganache with a spoon - you don't want it to be too stiff or too loose!
Using a handheld mixer, mix the chocolate ganache on medium speed for 2-3 minutes. It should become very light in color and fluffy.
Make Frosting
Melt the chopped chocolate in a microwave, stirring every 10 seconds so it doesn't burn.
In your stand mixer with the paddle attachment, beat the butter and confectioners’ sugar together on the lowest speed until combined.
Turn the speed to high for 2-3 minutes until light and fluffy.
Add in your cocoa powder and beat on low, then increase the speed to medium for 1-2 minutes.
Add your sour cream, melted chocolate and salt and beat on medium speed until it is spreadable and fluffy.
Assembly
Place one layer of the cake down and cover with half of the whipped chocolate ganache. Place another cake layer on top and repeat with the remaining ganache. Finally, place the third cake layer on top. Cover the sides and top of the cake with the sour cream chocolate frosting.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. Just click on 'METRIC' above the ingredients list to get the metric measurements. I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking.Butter: I use European style butter in my recipes. This will make your frosting much creamier and richer.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Buttermilk: If you don’t have buttermilk, replace it with an equal amount of sour cream or full fat yoghurt. You can also make your own buttermilk, for every cup of buttermilk add 1 tablespoon of lemon juice to 1 cup of milk and let sit for 5 minutes. Vegetable Oil: Use an unflavoured oil like rapeseed or canola oil if you don’t have vegetable oil. Two Layer Cake: if you prefer to have a two layer cake, use the same amount of ingredients but just bake in 2 8-inch cake pans, bake for 35-40 minutes.Chocolate Ganache: if it's cold where you live the whipped chocolate ganache will firm up quite a bit, so keep that in mind! Storage: Keep tightly wrapped in the fridge for up to 5 days. Let it come to room temperature before eating.
Making Ahead: You can make the cake layers ahead of time. Bake them and then once completely cooled, wrap in plastic wrap tightly. Store in the fridge for up to 2 days or in the freezer for up to 2 months. If frozen, allow to defrost and come to room temperature before frosting the cake.