Spiced Whole Orange Cake with Orange Mascarpone Icing

Name: Spiced Whole Orange Cake with Orange Mascarpone Icing
Ingredients: 1 large, thin-skinned orange; 50g (1¾oz) caster sugar; 1 small, thin-skinned orange (such as Valencia); 275g (9¾oz) self-raising flour; 2 tsp baking powder; 275g (9¾oz) caster sugar; 275g (9¾oz) baking spread, from the fridge; 4 free-range eggs; 1 tsp ground cinnamon; 1 tsp mixed spice; 50g (1¾oz) butter, softened; 175g (6oz) icing sugar, sieved; 250g (9oz) full-fat mascarpone cheese; 2 tbsp orange pulp, reserved from above