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 two 7" (18 cm) round cake tins. Alternatively, you can use two 8" cake tins or 3 6" 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 tins and bake for 25-30 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 raspberry 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 the seeds and make it smooth and beautiful. Cover it with cling film and leave it to cool in the fridge.
for pistachio butter
In a small, sharp food processor, mix shelled, salted and roasted pistachios for approx. 10 minutes or until it transforms into a butter/paste. If it seems like it's not coming together, add a teaspoon or two of vegetable oil to help it combine into a smooth paste. DO NOT ADD WATER.
for pistachio 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 and bubbles form on the top, cook for another 3-4 minutes.
Once it's done, transfer it to a bowl (you can strain through a sieve if you're afraid of possible lumps) 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 pistachio butter and mix until everything is combined.Cover with cling film and leave at room temperature until you're ready to assemble the cake.
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. Using a piping bag fitted with the Wilton 6B french star tip, pipe blobs around it to make a border. Fill it some raspberry jam and cover with a little bit of buttercream to stop the next layer from sliding.
Place the next layer on top and repeat the process. Only this time, when you drizzle raspberry jam on top, cover it with fresh raspberries.
Chill the cake for a half an hour to an hour just so everything has time to set.
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 22 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.