Sift flour, cocoa powder, baking soda and salt into a bowl. Set aside.
Add butter and both sugars into a bowl. Using a hand mixer, whisk everything until creamy. Then add in eggs 1 at a time, mixing well after each addition.
Add in sunflower oil, vanilla extract and mix until well combined. Now we will add the sifted flour mixture alternating with milk in 2 additions, mixing just to combine. Don't over mix.
Divide the batter evenly into prepared pans. Use a kitchen scale to be more precise. In my case each pan with raw batter weighs 16 oz (445 gr). Using the back of a spoon, spread the batter evenly.
Bake in a preheated oven at 350ºF (175ºC) for 35 minutes (if the cakes are baked individually) or until fully cooked. Bake for a slightly longer time if you bake them all together.
Cool for 10 minutes, remove from the pans and leave to cool completely on a rack.
NOTE: if you plan to frost the cakes the next day, wrap the cooled cakes tightly with cling films to avoid them getting dry.
The Sour Cream Chocolate Frosting
Chop chocolate into small pieces and add into a bowl together with butter and salt. Mix instant coffee with hot water until coffee is dissolved and add it into the bowl also.
Melt everything together using double boiler method. Use a spatula and mix now and then to avoid chocolate getting burnt. Once everything melted, remove from the heat and leave to cool for 5 minutes.
Add in sour cream and mix until well combined.
NOTE: this sour cream frosting will be soft at first (not the consistency to frost cakes) but it will harden as time passes. So leave until it gets to your desired consistency, mix now and then. I left it around 10-15 minutes.
Put frosting in a piping bag to make it easier.
Optional: I slice off the top of the cakes a bit to discard the crusty part. You don't need to do this if you don't want to.
Spread some frosting on a cake stand to help make the cake stay put. Place 1 cake on top. Pipe frosting on top. Repeat with the rest of the cakes, finishing with the last cake unfrosted on top.
Insert a wooden skewer on top in the middle of the cake and press until the bottom. The skewer helps to prevent the cakes to slide while we frost them.
Starting from the bottom, pipe the frosting all around the cake until you cover all the sides. Remove the skewer and continue frosting the top.
Take a long spatula (it's what I have) or a scrapper and run it along the sides, making the frosting even. Do the same for the top.
Using an offset spatula, make random half swirls all over the frosting to create that rustic look.
NOTE: I place my cake stand on a rotatable wooden pizza board and rotate it as I frost the cakes :D
How To Serve
This cake is rich and decadent. A small slice is more than enough. I love to serve it the next day as I feel it taste better.
How To Work With Sour Cream Chocolate Frosting
This frosting has the same manners as chocolate ganache that hardens in time. Read my post above on tips and how to work with it.
How To Keep
Keep in an airtight container at room temperature to avoid cake getting dry.
Since I'm using a high cake stand, no containers can fit it all :D So what I do is wait a bit until the frosting fully sets. Then I loosely cover the whole cake with cake stand (yes, I know) with cling films. I cover it loosely so that I don't mess with the decoration but I do make sure the cling films are overlapping so no air gets in.