Heat oven to 180C/fan 160C/gas 4. Butter two 20cm round sandwich tins and line the bases with baking parchment. Tip the sugar into a mixing bowl and add the softened butter and orange zest. Beat for 1 min or so until pale and fluffy, then beat in the egg yolks. Sift the flour and baking powder over the cake mixture and fold in lightly. Finally, fold in the almonds and orange juice. Whisk the egg whites until they just hold their shape. Fold one-third of the egg white into the cake mix, using the whisk. Repeat with another third, then the final third. Take care not to over-mix or you will lose the lightness of the egg whites.
Divide the cake mix between the prepared tins and bake for 30-35 mins. Cool in the tin for 5 mins, then turn out onto a wire rack, peel off the lining paper and leave to cool.
To make the icing, melt the chocolate over a pan of barely simmering water or in the microwave on High for 1½ mins, stirring every 30 secs, then leave to cool. Whip the creme frache until thick, then fold in the white chocolate. Set one cake on a serving plate or cake stand. Spoon half the icing onto the cake and top with the other cake. Spread with the rest of the icing, then allow to set in the fridge for at least 1 hr.
If you want to make the decorative curls, melt the chocolate in a bowl over simmering water or in a microwave and spread thinly over a baking tray. Leave the tray in the fridge for the chocolate to harden a bit, then use a fish slice or angled knife blade to scrape chocolate curls. Pile the curls in the middle of the cake and serve cut into slices.