Preheat your fan oven to 160°C (325°F) or 175°C if you're not using a fan oven. Grease and line with parchment paper three 6" round cake tins. Alternatively, you can use two 8" cake tins.
In a medium bowl sift together flour, cocoa powder, baking powder and salt. Add sugars, whisk until combined and set aside.
In a small bowl or a measuring jug, whisk together eggs, vegetable oil, sour cream and vanilla extract. These are your wet ingredients.
In a small pot, bring your water to a boil or simmer, it's important that it's hot. Add the wet ingredients to dry ones, stir a little bit, it'll be somewhat dry. Then add the boiling water and whisk until combined. I usually add the water in two stages to minimize clumps forming.
Divide the batter evenly among the 3 tins and bake for 23-25 minutes or until skewer inserted in the middle comes out with few moist crumbs. Do not overbake it.
Once they're done, let the cakes cool for 10-15 minutes on a wire rack before removing them from the pans to cool completely.
chocolate ganache filling
Place finely chopped chocolate in a bowl. Bring heavy cream and butter just barely to a simmer and pour over chopped chocolate. Cover it with a plate and leave it 1-2 minutes. Stir with a spatula until combined and smooth. Alternatively, you can do this in a microwave for 30 seconds.
Cover with cling film, leave it at room temperature to thicken to a spreadable consistency before assemblying the cake.
Chocolate Ermine Buttercream:
In a small saucepan (I always use heavy-bottomed one to prevent burning) combine flour, sugar, salt and milk. Cook over medium-high heat, stirring constantly with a whisk until it thickens. This can take about 15 minutes. The moment you realize it's thicker, cook for another 3-4 minutes.
Once it's done, put a cling film over it touching the surface to prevent "skin" forming and let it cool to room temperature. This is your pudding base. You can do this in a fridge, but before using it let it come to room temperature.
Using a stand mixer fitted with the paddle attachment, beat the butter for 5 minutes until it's pale and fluffy. Then add spoonful by spoonful of previously made pudding. Mix until it's all nicely incorporated and with no lumps. Add vanilla extract with cooled melted chocolate and mix until everything is combined. You might need to scrape the bowl a couple of times to ensure the chocolate is well incorporatedd and there are no white streaks.
Cover with cling film and leave it at room temperature until assemblying the cake. You'll probably have some leftover ganache, but it's hard making a smaller quantity than this.
Using a cake leveler or a long serrated knife level the tops of your cakes if needed.
In the middle of your cake board or a serving plate put some buttercream to stop the cake from moving around.
Place one cake layer on a cake board or a serving plate. Using a small offset palette knife or a piping bag, spread the frosting over the cake layer and make a dam around it. Pour ganache in the middle, level it with a palette knife. Top it with the next cake layer and repeat the process.
Place final layer on top, upside down, and do a thin crumb coat on the cake. Chill for 20-30 minutes in the fridge before final decorating.
Once cooled, frost and smooth the top and sides of the cake with remaining frosting. Then, using the back of your small offset palette knife, make swirls until you're satisifed with the "rustic" look. Add some strawberries on top to add interest and enhance the flavour.
If you don't have a stand mixer, you can use a hand mixer.
Not all ovens are the same. I suggest to check the cakes at 18 min mark and then adjust the baking time accordingly. Get to know your oven!
If you still don't own one, I advise you to buy a kitchen scale - they're cheap and easy to use, and always guarantee the same results in baking!
Cake is best eaten at room temperature. You can keep it in a fridge, wrapped in cling film, but take it out of it at least half an hour before serving.