Preheat the oven to 180C/350F/Gas 4.
For the baked chocolate mousse, line the base and sides of a 20cm/8in spring-bottomed cake tin with greaseproof paper. Lay the slices of fennel over the base of the tin.
Place the chocolate and butter into a heatproof bowl over a pan of gently simmering water and heat, stirring occasionally, until melted and well combined. Remove from the heat and allow to cool slightly.
Place the egg yolks into a bowl with two tablespoons of the sugar and whisk together. Stir in the melted chocolate and mix well.
Place the egg whites into a separate clean bowl with the remaining sugar and whisk until stiff peaks form when the whisk is removed.
Fold one-third of the egg white mixture into the chocolate mixture, to loosen it, then gently fold in the remainder and pour the mixture into the cake tin.
Place on the middle shelf of the oven and bake for 20 minutes, until risen and set. Remove from the oven and allow to cool before serving.
Meanwhile, for the candied fennel, place the fennel into a pan with the sugar and water and bring to the boil. Reduce the heat and simmer for 20-30 minutes, until the fennel is softened and the liquid reduced to a syrup. Remove from the heat and allow to cool.
To serve, cut the mousse into wedges and dust with icing sugar and cocoa powder. Serve with the candied fennel, a spoonful of whipped cream and a sprig of fresh mint.
