Preheat the oven to 200C/400F/Gas 6.
Melt 150g chocolate (for the mousse and the sauce) in a glass bowl over a pan of gently simmering water. The water should not touch the bowl.
Grease two chefs' rings and place on a baking sheet.
In a food processor, blend together the butter, egg and flour. Add the milk and blend again to make a smooth batter.
Add half the melted chocolate and gently fold in.
Fill the chefs' rings with the chocolate mixture. Sprinkle over the orange zest and bake in the oven for 8-10 minutes. They stay soft in the centre.
Once cooked, remove from the oven. Carefully remove the ring moulds, dust with cocoa and place onto plates.
For the sauce, add the sugar and a splash of boiling water to the remaining melted chocolate. Stir well.
For the orange cream, gently whip the cream and fold through the orange zest.
To serve, circle the mousse plates with chocolate sauce. Top the mousses with the orange cream and mint garnish.