Beat the soft butter, icing sugar and salt together until smooth. Set aside for now.
Very finely chop the dark chocolate and place it in a bowl.
Heat the double cream in a saucepan on low heat until piping hot and small bubbles start to form around the edge. Stir every so often so the cream doesn't burn.
Pour the hot cream all over the chopped chocolate and cover with a plate to lock in the heat for 2 minutes.
Use a whisk to gently stir the chocolate until it melts into a glossy ganache**.
Let the ganache cool for 10 minutes, then add it to the buttercream and mix well until smooth.
Spoon the ganache frosting into a piping bag fitted with a small open star nozzle.