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.
For the Berry Jam:
Cook frozen fruits with sugar over a medium-high heat until it reaches boiling. Turn the heat down and let it simmer for approx. 15 minutes or until it thickens. It's better to be thicker than looser, the plan is to lose a lot of water content and have concentrated flavour.
Strain the jam through a sieve to get rid of berry seeds and make it smooth and beautiful. Cover it with cling film and leave it to cool in the fridge.
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 berry jam and mix until everything is combined. Be careful not to add too much jam, otherwise your buttercream might curdle.
White 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. Alternatively, you can do this in a microwave for 30 seconds.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 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 and drizzle some berry jam over the frosting. 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, leaving some for rosettes on top.
Chill the cake for a half an hour to an hour before putting the chocolate drip on top. It has to be cold so the drip doesn't go all the way down.
Using a piping bag fitted with your favourite nozzles (I used Wilton 1M, 4B and 2D) pipe some rosettes on top after the ganache has set.