Preheat your oven to 350°F/175°C. Butter or grease two 8 inch baking pans and line it with parchment paper.
Combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt in the bowl of a stand mixer, using the paddle attachment. Set aside
In a large jug, combine the buttermilk, oil, eggs, and vanilla extract.
While the mixer is on low speed, slowly pour in the ingredients from the large jug
While the mixer is still running, slowly add the hot water and mix. Your batter will be very thin. Make sure the mixer is on your lowest speed so it doesn't splash everywhere
Pour the batter into the prepared pans. Bake for 35-40 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 Frosting
Melt your 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 on low speed until well 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 mix once more.
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. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method. Just click on ‘US Customary’ above the ingredients to see the cup 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, like Kerry Gold, 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. 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.