Place the elderflowers in a bowl. Heat milk to hot and pour over the flowers. Let stand at least one hour, then strain, pressing the liquid out of the flours. Discard flowers.
Preheat oven to 180 degrees C. Grease a springform cake tin.
Cream butter and sugar until light and fluffy. Beat the egg then stir into the butter mixture.
Sift the flour together with the salt and baking powder and cornflour, then mix in the polenta. Quickly stir in the elderflower milk.
Pour batter into springform tin and smooth out. Bake 40-50 minutes, until it tests clean.