Preheat your oven to 200°C/gas mark 6. Grease and line the base of a 20cm springform cake tin.
Put the butter, yogurt and sugar in a bowl and cream together until light and fluffy.
Blitz the pistachios to dust in a food processor – don’t blitz them too much, though, or they will turn to butter. Add the blitzed pistachios, polenta, baking powder and lemon zest and juice to the butter mixture and mix well. Then crack in the eggs, one by one, and mix in.
Pour into the cake tin and bake for 45-50 minutes, until a skewer comes out clean. Remove from the oven and leave to cool in the tin. Make a few holes in the warm cake with a skewer, then gently pour the elderflower cordial slowly over the cake, allowing it time to seep in. Leave the cake in the tin until cool enough to transfer to a cooling rack.
For the icing, mix the yogurt, icing sugar or honey and elderflower cordial until smooth. Spread over the cooled cake and top with the pistachios.