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To make the cake, put the orange in a saucepan, cover with water, bring to the boil and simmer for 1 hr or until you can pierce the orange easily with a knife.
Drain, leave to cool, then cut the orange into quarters. Cut each quarter in half so that you have 8 pieces, and remove any pips. Put 75g of the sugar in a small pan and add 100ml water and the orange pieces. Heat gently until the sugar has dissolved, then turn up the heat and simmer for 10 mins. Allow the orange to cool slightly, then either blitz with a stick blender or liquidise to a pulp. Crush the coriander seeds using a pestle and mortar or a spice grinder, then add to the warm orange pulp along with the polenta. Stir to combine, then leave to cool.
Heat oven to 190C/170C fan/gas 5. Grease and line the base and sides of a 22cm loose-bottomed cake tin. Put the butter, flour, baking powder, eggs and the remaining 100g caster sugar in a large mixing bowl along with the cooled orange pulp. Mix until thoroughly blended using an electric hand mixer. Spoon into the prepared tin and bake for 35-45 mins or until springy to the touch – a skewer inserted into the centre should come out clean. The cake should be pale in colour and just leaving the sides of the tin. Transfer to a wire rack and leave to cool.
Meanwhile, prepare the decoration. Using a potato peeler, peel half the zest from the orange into large strips, then slice very thinly so that you have lots of orange zest shreds (like you would for marmalade). Put the caster sugar and 100ml water in a small saucepan set over a low heat until the sugar has dissolved, then add the orange shreds. Bring to the boil, then simmer for 8-10 mins until the shreds are quite soft. Remove using a slotted spoon and set aside on a tray lined with baking parchment.
When the cake has cooled, sift the icing sugar into a small bowl. Juice the orange and add enough of it to the icing sugar to make a runny icing. Drizzle the icing over the cooled cake and scatter the orange shreds on top.