Carefully pick off all of the elderflowers from the stems, discarding as much of the green as possible and only using the petals. Place the elderflower in a bowl then pour over the warm milk and leave to steep for at least 1 hour.
After an hour, run the elderflower milk through a fine sieve and discard the contents of the sieve.
Preheat the oven to 180 C / Gas 4. Grease a deep 18cm springform cake tin.
Combine the butter and sugar in a mixing bowl and beat until fluffy. Beat in the egg until the mixture is pale and creamy.
Sift in the flour and mix with salt, baking powder, polenta and cornflour. Gradually mix everything together adding the elderflower milk slowly. Transfer the cake mixture into the prepared cake tin.
Bake in the preheated oven for 40 to 50 minutes, until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool completely on a cooling rack. Once cool remove from the tin and transfer to a serving plate. Dust with icing sugar and serve.