Preheat the oven to 180°C.
Line a 20cm spring-bottomed cake tin and arrange fennel slices across the base.
Melt chocolate and butter in a heatproof bowl over simmering water until combined.
Whisk egg yolks with 2 tablespoons of sugar and stir in the melted chocolate.
Whisk egg whites with the remaining sugar in a clean bowl until stiff peaks form.
Fold one-third of the whites into the chocolate, then gently fold in the rest and pour into the tin.
Bake for 20 minutes until risen and set, then allow to cool.
Simmer fennel with sugar and water for 20 minutes until softened and syrupy.
Serve mousse wedges dusted with sugar and cocoa alongside candied fennel, cream, and mint.