Line a 20-23 cm (8-9 inch) cake pan with circle of baking paper. Grease very well the pan on top of the paper.
In a mug combine cocoa powder and hot water. Whisk until smooth.
In a medium bowl combine about half of the sugar, flour and salt.
In a large mixing bowl beat egg whites with cream of tartar (lemon juice) until soft peaks form. Add the remaining sugar and beat until very stiff peaks form.
Remove one cup from the egg whites and add it to the cocoa mixture. Whisk until homogeneous.
Using a rubber spatula gradually add the flour mixture into the remaining egg whites, folding carefully. When the flour is absorbed fold the cocoa mixture into the batter until the colour becomes uniform.
Transfer the batter into the prepared pan and smooth the top with a spatula. Bake about 40 minutes or until a wooden skewer inserted in the centre comes out clean.
Let the cake cool for about 10 minutes in the pan and then run a knife around its edges. Remove from the pan and let cool until room temperature before assembling the cake.
As this cake has a very spongey texture, it is recommended to refrigerate it before cutting it in layers in order to make the process easier.
In a large mixing bowl using an electric mixer beat mascarpone and sour cream (make sure both are at room temperature). Gradually add the melted chocolate and continue beating at a lower speed of the mixer until the frosting becomes stiff.
Refrigerate for 1 hour before using it.
To assemble the cake:
Using a serrated knife cut horizontally the cake in two equal layers. Place one layer on a serving plate and cover with about ⅓ of the frosting. Place the second layer. Spread the remaining frosting all around the cake, making sure it is as even as possible.
Refrigerate the cake for at least 4-6 hours before serving. Decorate with chocolate balls and chocolate coated dried fruits.
Adapted from The Cake BibleIf you don't feel confident to bake this cake without any raising agent, you can use self raising flour instead of all-purpose.