Preheat your fan oven to 165°C (320°F). Grease and line with parchment paper three 6 inch ( 15 cm) round cake tins.
In a medium bowl mix sifted cocoa powder with hot water. Whisk until combined and smooth. Leave it aside to cool to room temperature.
Whisk together sifted flour, baking powder and salt. Set aside. Add sour cream, melted chocolate and vanilla extract to cocoa mixture. Whisk together until combined. These are your wet ingredients.
Using a stand mixer fitted with a paddle attachment, cream butter and sugar on medium-high until pale and fluffy - approximately 3 minutes. Stop the mixer and scrape down the bowl. Turn the mixer to medium-low speed and add eggs one at a time - making sure it's fully incorporated after each addition. Stop the mixer and scrape down the bowl.
Add your dry and wet ingredients, alternating between one another, starting and finishing with dry ones. (3 additions of dry and 2 of wet ingredients)
Divide the batter evenly among the 3 tins and bake for 25 minutes or until skewer inserted in the middle comes out mostly clean.
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.
In a small saucepan combine frozen cherries and sugar and bring to boil over medium heat. Simmer for a few minutes.
Dilute the corn starch with water and pour over the cherries mixing constantly with a large wooden spoon until everything is incorporated.
Bring the mixture to a boil and cook for a few minutes. If the filling is too thick you can add some more water. That depends on the water content of frozen cherries.
Once thickened, transfer it to a bowl and cover with cling film touching the surface to prevent "skin" forming.
You can put it in the fridge to cool completely.
Put the flour, sugar, salt and milk into a small heavy bottomed saucepan and combine using a whisk.Cook over medium to high heat, whisking continuously, so it doesn't burn or catch in the bottom. Once it starts to get thicker and you notice bubbling, cook for another 2 minutes, still whisking continuously, until it resembles pastry cream. This could take about 10 minutes.
When the pudding base is done, strain it through a sieve into a shallow plate, to get rid of any possible lumps. Cover it with plastic wrap touching the surface to prevent«skin» forming and let it cool to room temperature. You can speed up the process by letting it cool in the fridge, but prior to using it it must be 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 base. Beat everything until it's all nicely incorporated and with no lumps. Add vanilla extract and mix for another 30 seconds.
Chocolate Ganache Drip
Place finely chopped chocolate in a bowl. Bring heavy cream 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. Make it 10 minutes before pouring over the chilled cake so it has time to cool and thicken a bit.
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 keep the cake from moving.
Spread a thin layer of buttercream on top of the layer and pipe a border around the outside to hold the cherry filling in. Repeat with the next layer.
Place final layer on top and do a thin crumb coat on the cake. Chill for 20-30 minutes in the fridge before final decorating.
Frost the top and sides of the cake with remaining frosting.
Chill the cake for an hour before putting the chocolate drip on top. It has to be cold so the drip doesn't go all the way down.
If you don't have a standing mixer you can use a hand mixer.
Not all ovens are the same. I suggest to check the cakes at 20 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.