Preheat the oven to 180C/350F/Gas 4. Grease and line a 24cm/9.5in springform cake tin with greaseproof paper.
Scatter the fennel slices evenly over the base of the prepared cake tin.
Place the chocolate and butter into a heatproof bowl set over a pan of simmering water (make sure the bowl doesn't touch the water) and allow the chocolate to melt, stirring occasionally. Remove from the heat and set aside to cool slightly.
Place the egg yolks into a large mixing bowl with 2 tablespoons of the sugar and whisk lightly to combine.
Place the egg whites into a separate bowl and whisk until soft peaks form when the whisk is removed from the bowl. Gradually add the remaining sugar to the egg whites, whisking continuously, until firm peaks are formed when the whisk is removed.
Pour the melted chocolate into the bowl with the egg yolks and stir to combine. Fold one-third of the stiff egg whites into the egg yolk and chocolate mixture until combined, then gently fold in the remaining egg whites.
Pour the mixture into the cake tin and gently smooth over the top. Bake in the oven for 20-25 minutes, or until the cake is golden-brown and springy to the touch. The cake is done when a skewer inserted into the middle comes out clean.
Remove the cake from the oven and leave to cool on a wire rack.
Serve the cake in slices with a dollop of crème fraîche.
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