Preheat the oven at 180°C or gas mark four and butter and flour or line two 20cm/8in cake tins with parchment.
Make a strong coffee with 280ml milk and 8 tsp Nescafé (instant coffee). Allow to cool. I make all the coffee at once and then split it into half using one half for the icing and the other for the actual cake.
Melt the 50g butter for the icing and set aside to cool.
In a bowl, sift the flour together with the baking powder, soda bicarbonate, cacao powder and a pinch of salt unless you use salted butter. Mix a little.
In a freestanding mixer place the egg yolks with 200g sugar, vanilla and the soft butter. Mix for about 4-5 mins until it becomes nice, fluffy and creamy. Add the kefir or the soured cream and mix for another minute or so. Add the melted chocolate(90g) and mix for 1 more min.
Beat the egg whites with the remaining 80g sugar until stiff and form nice and firm peaks.
With a spatula, fold slowly the flour and cacao mixture into the egg and butter mixture. Add the coffee (make sure it is not hot but is lukewarm at most) and fold in the heated egg whites. Fold carefully and gently so the batter remains nice and airy.
Pour the cake mixture into the tins making sure they are getting pretty much even amounts of cake mixture.
Bake for about 30-35 min until risen. Check the middle section of the cake with a wooden skewer to make sure this comes out clean. That will indicate that the cake is ready. Take the cakes out of the oven let them rest for 10-15 min. Take out of the tins and gently place them on a cooling rack.
I always make my icing while the cake bakes. I recommend to set up a timer so that the cake does not get forgotten, thus overly baked.
Sift the icing sugar into a bowl, add 1/2 coffee liquid mix well so that no sugar lumps form. Add the remaining coffee, the vanilla, the melted butter and the melted chocolate. Mix all well until incorporated and looks lush and creamy.
Put in the fridge to rest and harden for about 15 min.
Ideally is to cut each cake into two and layer them with the icing in between each layer. I have not cut them this time and I should have so I recommend you do.
Finish with a top icing layer and enjoy!