Grease and line the bases of 2 x 20cm/8in round sandwich tins. Preheat the oven to 180°C, 350°F, Gas 4. Whisk the eggs and caster sugar together in a large bowl until thick enough to leave a trail when the whisk is lifted. Sift the flour into the bowl then add the grated chocolate and fold in with a metal spoon. Divide between the prepared tins and bake for 20- 25 minutes until just firm to the touch. Remove from the tins and cool on a wire rack.
Stir the rosewater into the crème fraîche and use to sandwich the cakes together. Put the white chocolate and butter in a heatproof bowl set over a saucepan of barely simmering water so the base of the bowl is not touching the water. Leave until melted then allow to cool and stir in the cream and icing sugar and beat until smooth.
Leave to cool further until it forms soft peaks then spread over the top and side of the cake. Decorate with chocolate curls, sugared rose petals and a light dusting of icing sugar.
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