Grease a Swiss roll pan with butter and line with baking paper.
For the sponge, place the eggs and sugar in a large bowl and, using an electric hand-held mixer, mix together for about five minutes, or until the mixture is white and thick.
Sift over the flour and fold in gently using a spatula, taking care not to over mix.
Pour the mixture into the prepared pan and level the surface.
Bake for 20 minutes, or until golden-brown and springy to the touch.
Cool in pan for five minutes then turn out onto a wire rack and leave to cool completely.
When the sponge is cold, carefully slice the cake in half horizontally, so you have two thin sponges of equal depth, then cut each of the sponges in half so you'll have 4 layers of sponge (one of mine broke and I only got to use 3)
Dissolve the coffee in the boiling water and add the brandy or the rum extract for a non alcoholic version. Set aside to cool.
Make the whipped cream. Using a hand mixer on high speed, mix heavy cream with 3 tbsp icing sugar until stiff peaks form. Do not over beat.
Mix the other egg yolks with 7 tbsp icing sugar, using a hand mixer. You should get a creamy, white mixture.
Add mascarpone cheese, bit by bit, util fully incorporated.
Add 1 tsp cocoa powder and melted chocolate and incorporate using a hand mixer on medium speed.
Fold in whipped cream, a little at a time, using a spatula.
Keep the filling in the fridge if not using immediately.
Line the cake pan again with baking paper, leaving some excess on the sides for lifting the cake later.
Place one layer of the sponge cake in the cake pan. Spoon over some of the coffee mixture then spread one quarter of the mascarpone frosting over the soaked sponge.
Repeat for the other layers.
Chill in the fridge for 1-2 hours then dust with cocoa powder.