In a saucepan heat the cream until it starts to boil; in the meantime, while the cream is heating, coarsely chop the chocolate with a knife.
In a bowl, add the cream and chocolate and mix them with a hand whisk until you get a smooth liquid cream, then add the butter, which will melt immediately, and stir again.
The cream thus obtained is nothing more than a simple ganache. To make it similar to a frosting, let it cool for about half an hour, then add the mascarpone and blend all, with the electric mixer, to obtain a light and frothy cream.
The doses of mascarpone are indicative: depending on the density of the cream you want to obtain, you can add more, or reduce the cheese.
When the mascarpone cream is ready, use it to fill and cover the cake with the help of a spatula.
The cake is kept in the fridge until half an hour before serving, and keeps delicious for 3 or 4 days (maybe even later, but mine didn't last longer!)