Combine the cream, icing sugar and cardamom in a small bowl. Cover with plastic wrap and place in the fridge.
Put the whole, unpeeled mandarins in a saucepan. Cover with cold water and bring to the boil over high heat. Drain. Repeat the boiling process two more times, then set the drained mandarins aside to cool.
Preheat your oven to 160C. Grease a 22cm round cake pan with the butter. Line the base and sides of the pan with non-stick baking paper.
Place the mandarins in a food processor and process until smooth. Then, with the motor running, add the eggs one at a time, until combined. Add the almond meal, caster sugar, baking powder and vanilla and process until just combined.
Pour the mixture into the cake pan and bake in the oven for one hour.
While the cake is in the oven, combine the extra sugar, mandarin juice and water in a small saucepan over low heat and stir until the sugar dissolves. Then, increase the heat and simmer for five minutes or until the syrup thickens slightly.
When the cake is cooked, pour 3⁄4 of the mixture over the hot cake. Then, set the cake aside to cool.
Serve with the cardamom cream and remaining syrup, along with a sprinkle of the extra cardamom.