Put the oranges cut side up into a medium pan, and add their own depth of cold water. Cover and simmer for 45 minutes, until completely tender. Drain, then blitz to a smooth purée using a stick blender or food processor. You should have 250g purée.
Preheat the oven to 180°C, gas mark 4. Grease 2 x 20cm deep tins and line the bases with parchment.
Combine the flour, almonds, bicarbonate of soda, spices and salt in a large bowl and whisk to combine. Melt 150g butter with the sugar and golden syrup until the sugar dissolves.
Tip the orange purée, butter mixture, milk, eggs and chopped ginger into the flour mix, then whisk until combined. Divide between the tins then bake for 25 minutes, until golden and a skewer inserted into the middle of the cakes comes out clean.
Cool in the tins for half an hour, then turn onto a wire rack lined with parchment and leave to cool completely.
Beat the mascarpone with 150g orange curd and 2 tbsp of the ginger syrup until creamy. Using a palette knife, sandwich the cakes with 1/3 of the frosting, then cover with the remainder.
To decorate, first stick rounds of ginger around the base of the cake. Sprinkle the top edge of the cake with a little ground ginger. Loosen 2 tbsp more curd with 2 tsp ginger syrup and drizzle it over the cake.