Preheat the oven to 180C (fan 160C), 350F, gas mark 4, with the middle shelf at the ready. Grease 2 x 20cm â¢ 8 in loose-bottomed sandwich cake tins with a little butter, line the bottoms with discs of baking parchment and sit them on a large baking sheet.
Put the butter, sugar, flour, sour cream, eggs, cocoa powder, baking powder and salt in a large bowl or in a freestanding electric mixer or a food processor. Split the vanilla pod open, scrape out the seeds and add them also (or the vanilla extract). Then mix or blend to give a smooth, soft mixture.
Divide evenly between the cake tins, smooth the tops and place in the oven for 25-30 minutes.
To check the cakes are baked, insert a skewer into the middle of each cake and if it comes out clean, then they are ready to come out. Remove them from the oven and leave to cool in the tins for a few minutes before carefully removing and leaving to cool completely on a wire rack (about 25 minutes).
When the cakes are almost cool, start making the buttercream. Break the chocolate into a medium bowl and melt it in the microwave in 30-second blasts, stirring in between. Otherwise, sit the bowl of chocolate on a medium pan of simmering water, making sure that the water does not touch the bottom of the bowl, as this may make the chocolate grainy.
Sift the icing sugar into the electric mixer or processor bowl (or large bowl if doing by hand). If I use a food mixer or processor for this, I donât worry about sifting it as the blade will do a good job of blending any lumps. Add the butter and milk (or water) and beat until it is really light and fluffy. You will need to do this like mad if blending by hand. Then pour in the melted chocolate, stirring all the time.
Sit one of the cakes on a serving plate or cake stand. I often put a little dollop of buttercream underneath the sponge so that the cake doesnât move around. Then put about a third of the buttercream on and spread it around. Then sit the other sponge on top and spread the remaining buttercream all over so it is completely covered. It doesnât need to be perfect. Doing this is a great excuse to get nice and messy.
Once you have covered the cake, and of course yourself, suitably in chocolate, take the Maltesers and stick them all over the cake.