Prepare the Cake Tin:
Grease and line the base of a 24cm springform cake tin with parchment paper.
Melt the Chocolate:
Break 120g of dark chocolate into pieces and melt gently over a double boiler or in the microwave. Stir until smooth, then set aside to cool slightly.
Combine Egg Yolks and Chocolate:
Once the melted chocolate has cooled for about 5 minutes, beat in the 5 egg yolks until fully incorporated.
Cream Butter and Sugar:
In a separate bowl, cream together 200g of softened butter and 100g of the caster sugar until pale and fluffy.
Add the chocolate and egg yolk mixture to the creamed butter and sugar, mixing thoroughly.
Whisk Egg Whites:
In a clean bowl, whisk the 5 egg whites to firm peaks. Gradually add the remaining 120g of caster sugar in three batches, whisking continuously until stiff and glossy.
Fold in Egg Whites and Flour:
Gently fold a third of the meringue into the chocolate mixture to lighten it.
Alternately fold in spoonfuls of the sifted plain flour and the remaining meringue, being careful not to deflate the mixture.
Bake the Cake:
Pour the batter into the prepared cake tin and level the surface.
Bake in a preheated oven at 160°C (fan 140°C) for 40-45 minutes, or until the top forms a light crust and a skewer inserted into the center comes out clean.
Remove from the oven and allow the cake to cool in the tin for 10 minutes before transferring to a wire rack to cool completely.
Prepare the Filling:
In a bowl, whip the double cream until thickened but not stiff.
Fold in the half-fat crème fraîche and the grated zest of the limes or lemon.
Assemble the Cake:
Once the cake has cooled, carefully slice it horizontally into two layers using a large serrated knife.
Spread the zesty cream filling over the bottom layer, then sandwich with the top layer.
Prepare the Ganache Icing:
Break 150g of dark chocolate into pieces and place in a saucepan with 142ml of double cream.
Melt over low heat, stirring until smooth. Allow to cool until it reaches a thick pouring consistency.
Ice the Cake:
Place the assembled cake on a wire rack over a large plate to catch any excess icing.
Pour the ganache over the cake, ensuring it covers the top and sides evenly.
Use a palette knife or the back of a spoon to smooth the icing.
Decorate:
While the icing is still setting, sprinkle the top and sides with the grated chocolate.
For an elegant touch, create chocolate curls by drawing a vegetable peeler down the side of a thick block of chocolate. Pile the curls in the center of the cake.
Serve:
Allow the ganache to set fully before serving.
If prepared in advance, let the cake come to room temperature before serving to enhance its gooey texture.