Start this recipe by greasing and lining a 7inch/17cm cake tin. Mix together the flour, sugar, baking powder & mayonnaise. This is best done by an electric food mixer to ensure the mixture is thoroughly mixed. Dissolve the cocoa powder in the boiling water and stir into the mixture, along with the essence and mix until thoroughly blended. Pour the cake mixture into the prepared cake tin and even out. Bake in the oven at 180`c for 30-40 minutes, or until cook through. Remove from the oven and allow to cool down in the cake tin before removing. Meanwhile, prepare the orange and white chocolate frosting by beating together the butter, icing sugar and the orange zest. Finally, beat in the melted white chocolate till well combined. Cut the cooled cake in half horizontally and use ½ of the frosting to sandwich the cake together. Use the remaining frosting to decorate the top of the cake and finish off with some more orange zest.