In a saucepan over medium heat, bring the cream to a gentle boil; in the meantime, coarsely chop the chocolate with a knife.
300 g whipping cream
Take the saucepan from the stove and stir chocolate and butter into the cream, like you would if you were making a simple ganache.
50 g butter, at room temperature, 300 g dark chocolate
Let the ganache cool for about half an hour, then whisk in the mascarpone (manually or in your stand mixer).
250-300 g mascarpone cheese
The amount of mascarpone is indicative: you can add more or less, depending on the desired frosting texture.
When the mascarpone frosting is ready, you can use it to fill and cover the cake.
Store the cake in the fridge until half an hour before serving. It will keep well for 3-4 days (maybe longer, but mine didn't last!).