Beat the softened butter until it becomes pale and creamy.
Add i the icing sugar and ground pistachios and beat gently until just incorporated and smooth and spreadable.
Spread a small amount on to a cake board or cake plate and place the first sponge layer on top.
Fill a piping bag with a round nozzle and pipe just around the outer rim of the sponge layer.
Heat the cream until almost simmering then stir in the chopped chocolate.
Keep stirring until the chocolate has completely melted and its smooth and glossy.
Allow the ganache to cool stirring regularly until its spreadable.
Spoon some of the ganache on top of your base layer of sponge cake up to the piped ring of buttercream.
Sprinkle half of the chopped pistachios over the ganache.
Place the second layer of sponge on top of the buttercream/ganache and press down gently.
Spoon a third of the buttercream on to the top of the cake then, using a large spatular, spread the buttercream until smooth on top, and gently ease it down the sides of the cake.
Once the whole cake is covered in one smooth layer, place the cake in the fridge for 15 minutes.
Add a second layer of buttercream and make sure the edges are smooth.
If the ganache has become firm while you've been icing, gently reheat it, stirring frequently to make sure it stays smooth and glossy.
Using a teaspoon, spoon drips over the edge of the cake.
Once you have created drips round the whole cake, spoon the ganache on top and gently ease right up to the edge of the cake until smooth.
To finish, sprinkle over the remaining chopped pistachios.