Pre-heat the oven to 200`c. Grease and line a small loaf tin or 20cm/8in round tin. Cream the butter and sugar with an electric whisk in a large mixing bowl until very pale and fluffy. Add the beaten eggs, a little at a time, whisking well after each addition. Then whisk in the vanilla essence and citrus zest. Sift the flour into a large bowl, then fold it gently into the butter and egg mixture. Divide the cake batter amongst the 15 muffins cases, filling almost to the top. Bake for 25 minutes or until the cup cakes are springy to the touch. Allow to cool slightly, then turn out onto a wire rack to cool completely. Meanwhile, prepare the frosting by beating together the butter, icing sugar and grated citrus zest. Finally, beat in the melted white chocolate till well combined. Pipe the frosting onto the cooled cup cakes and serve.