Peel long strips from the large orange and boil in water with 25g sugar for one minute.
Drain peel, toss with remaining sugar, and leave to dry until crisp.
Grease and line two 20cm sandwich tins and preheat oven to 180°C.
Boil the whole small orange in water for 30 minutes until soft, then cool.
Remove pips and blend the whole orange in a food processor until medium-chunky.
Reserve 2 tablespoons of orange pulp and set the rest aside in a bowl.
Blend remaining cake ingredients in the processor until smooth, then stir in pulp.
Divide mixture between tins and bake for 30 to 35 minutes until well risen.
Cool cakes on a wire rack and remove the baking paper.
Whisk butter and icing sugar until smooth, then beat in mascarpone and reserved pulp.
Slice cakes to create four layers and stack with icing between each layer.
Top with remaining icing and candied orange rind.