For the decoration, using a canelle knife, peel thick long strips of orange peel from the large orange. If you don't have a canelle knife, carefully slice away the rind and then cut into pencil-thick strips (use the leftover orange for juice or a fruit salad). Place in a saucepan, cover with boiling water, add half of the sugar and boil for one minute, drain then place on non-stick paper. Scatter over the remaining sugar and toss together, leave in a warm place to dry out for as long as possible or overnight until crisp.
Grease and line the bases of two 20cm/8in sandwich tins with non-stick baking paper. Preheat the oven to 180C/350F/Gas 4 (fan 160C).
For the cake, place the whole orange in a small saucepan, cover with boiling water and boil for 30 minutes, or until soft. Leave to cool. When the orange is cold, cut in half and remove any pips.
Roughly chop the orange and blend, including the skin, in a food processor until medium-chunky in texture. Transfer to a bowl and set aside, reserving two tablespoons for the icing.
Add the remaining cake ingredients to the food processor and blend until just smooth (be careful not to over-beat the mixture). Remove the blade, carefully stir in the pulp that you have set aside, then divide between the two prepared tins and level the tops.
Bake for 30-35 minutes, or until well risen, lightly golden-brown and shrinking away from the sides of the tins. Leave to cool for a few minutes and then transfer to a wire rack to cool completely. Peel off the baking paper.
For the icing, place the butter in a mixing bowl and, using an electric hand whisk, gradually beat in the icing sugar until smooth. Add the mascarpone and whisk together until smooth and creamy. Add the reserved orange pulp and mix until smooth.
Slice each cake in half to make four layers. Divide the icing into four and then stack the layers of cake - icing between each layer and finishing with icing on the top.
Decorate the cake with the candied orange rind and serve.