Grease two 7 in (18 cm) sandwich tins and line the bottoms with a disc of greased greaseproof paper. Sift the flour, baking powder and salt together two or three times and put on one side. Put the eggs and the sugar into a deep bowl over a pan of hot water and whisk with an electric or hand whisk until the mixture becomes really thick so that it retains the impression of the whisk when you pull it out. Add the rose or violet essence and whisk in. Take the bowl off the pan of hot water. Gently fold in the sifted flour with a metal spoon. Divide the mixture equally between the two sandwich tins and cook in the centre of a pre-heated oven at Gas 5 (375°F/190°C) for 15 minutes until the cakes are light brown and springy to a touch in the centre. Lay a folded teacloth on a wire cooling rack. Loosen the edges of the cakes with a knife and turn out on to the teacloth. (This avoids getting the marks of the wire rack on the cakes.) Carefully remove the greaseproof paper. When cool, whip the cream until stiff, adding a few drops of rose or violet essence to taste, and use to sandwich the cakes together. Sprinkle the top with caster sugar and decorate with crystallised rose or violet petals.