Preheat the oven to 180°C (350°F), Gas Mark 4 and grease a 20cm diameter springform tin, then line the
base with baking parchment.
Bash the cardamom pods in a pestle and mortar and extract the seeds. Then bash the seeds to a fine powder and add to a bowl together with the ground almonds, polenta and baking powder.
Beat the sugar and butter in a bowl until the mixture is light and pale. Add the eggs, one at a time, beating well after each addition.
Tip the bowl of dry ingredients into this mixture and fold with a spatula until just combined. Add two-thirds of the orange zest together with the vanilla extract, and just fold through.
Pour the mixture into the prepared tin and place it on the middle shelf of the oven to bake for about 40 minutes or until a skewer comes out clean. Remove from the oven and the tin and allow to cool on a wire rack.
Prepare the syrup by placing all the ingredients in a small saucepan over a medium heat and bringing to a steady simmer.
Pierce holes all over the cake with a wooden skewer while it is cooling and pour over half the syrup, a little at a time, until the cake has soaked it up.
Sprinkle with the pistachio nuts, drizzle with the remaining syrup and sprinkle with remaining orange zest to decorate. Serve in slices with a little crème fraîche.