Combine the cream, icing sugar and cardamom in a small bowl. Cover with plastic wrap and place in the fridge.
Place mandarins in a saucepan. Cover with cold water. Bring to the boil over high heat. Drain. Repeat 2 more times. Set aside to cool. Quarter, leaving peel intact. Remove and discard any seeds.
Preheat oven to 160°C Brush a round 22cm (base measurement) cake pan with melted butter to grease. Line the base and side with non-stick baking paper.
Place the mandarin in the bowl of a food processor. Process until smooth. With the motor running, add the eggs, 1 at a time, until combined. Add the almond meal, caster sugar, baking powder and vanilla. Process until just combined. Pour mixture into the prepared pan. Bake for 1 hour or until a skewer inserted into the centre comes out clean.
Meanwhile, combine the extra sugar, mandarin juice and water in a small saucepan over low heat and stir until the sugar dissolves. Simmer for 5 minutes or until the syrup thickens slightly.
Pour three-quarters of the hot syrup over the cake. Set aside for 20 minutes to cool slightly. Transfer to a serving plate. Cut the cake into slices. Divide among serving plates and drizzle over the remaining syrup. Top with a dollop of the cream mixture and sprinkle with extra cardamom to serve.