Preheat your oven to 350°F/175°C. Butter or grease two 8-inch cake pans and line it with parchment paper.
Combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl with a whisk.
¾ cup cocoa powder, 1 ¾ cups granulated sugar, 2 cups all purpose flour, 2 teaspoon baking soda, 1 teaspoon baking powder, ¾ teaspoon kosher salt
In a large jug, whisk together the buttermilk, oil, eggs, and vanilla extract.
¾ teaspoon vanilla extract, 1 cup buttermilk, ½ cup vegetable oil, 2 eggs
Slowly pour in the ingredients from the large jug and mix until just combined.
Slowly add the hot water and mix. Your batter will be very thin and it makes a lot of cake batter!
1 cups very hot water
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
Chop up your chocolate and place in a large bowl.
18 oz dark chocolate
Heat the heavy cream in a saucepan over medium-low heat until it is simmering. Make sure it does not boil!
2 ¼ cups heavy cream
Pour the hot cream over the chopped chocolate and let sit for 5 minutes.
Stir the chocolate and cream mixture with a rubber spatula until completely smooth, then stir in the salt.
pinch salt
Let sit at room temperature for 2-3 hours until set. If it's too hot when you're making this you can place it in the fridge, but stir it every 15 minutes or so! If you don't whisk, you'll risk lumps in the ganache that will get stuck in the piping bag later.
Once the ganache has set and is firm but still soft, use a handheld electric mixer to whisk until light and fluffy.
Assemble your cake! Place one cake layer down, then cover in some of the whipped ganache. Place the second layer on top, and cover the sides and top with remaining frosting. Pipe using a 1M wilton tip if you want swirls on top of the cake.
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.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 yogurt. You can also make your buttermilk, for every cup of buttermilk add 1 tablespoon of lemon juice to 1 cup of milk and let sit for 5 minutes. Amount of Ganache: If you make the amount listed, you will have enough to frost the cake and do some piping on the edges. If you want even more chocolate ganache, then use 3 cups of heavy cream and 24 oz. chocolate. Chocolate: I recommend using dark chocolate (around 60-70% cocoa) or semisweet chocolate. Milk chocolate and white chocolate won't work with the amounts listed because the ratios will be off. I don't recommend using chocolate chips because those contain emulsifiers. Vegetable Oil: Use an unflavoured oil like canola oil or sunflower 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 them 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.