Preheat the oven to 170°C. Arrange the cupcake cases in a muffin tray.
Sift the castor sugar, flour, cocoa powder, baking powder and salt into a mixing bowl.
Add the eggs, sunflower oil, buttermilk and vanilla extract. Mix until combined. Stir in the melted chocolate.
Divide the mixture between the cupcake cases. Bake for 20-25 minutes, or until an inserted skewer comes out clean. Transfer the cupcakes to a cooling rack and leave them to cool completely.
To make the ganache: chop the dark chocolate into small pieces and place in a small heatproof bowl. Heat the cream and butter until the butter has melted and the cream is just starting to bubble.
Take the cream off the heat and pour over the chopped chocolate. Leave to stand for a minute, then whisk until smooth. Stir in the whiskey.
Leave the ganache to cool to room temperature. Whisk the ganache with an electric beater for a minute. Spread the ganache over the cooled cupcakes and decorate with chocolate curls or grated chocolate.