Crush the biscuits and combine with the butter. Place a 20 cm diameter ring mould onto a baking sheet covered with baking parchment. Tip the biscuit mixture into the ring mould and, using the back of a spoon, press down to a thickness of 5 mm.
Bring the cream to the boil, pour onto the couverture chocolate and whisk until smooth. Add the egg, Cream Cheese, fromage blanc, sugar and salt.
Stir until smooth and any lumps have disappeared but do not emulsify. Pour onto the biscuit base then bake in the oven at 120°C for 20 minutes (the cheesecake should have a slight wobble in the centre). Remove from the oven. Cool for 30 minutes at room temperature then refrigerate for one hour. Remove the ring mould and position on a cake board.
Bring the cream to the boil, add the finely chopped mint, finely grated lime and sweetened condensed milk and infuse overnight in the refrigerator to allow the flavours to fully develop.
The next day, whip the Chantilly and transfer to a piping bag fitted with a Saint-Honoré nozzle.
Spread a thin layer of marmalade onto the cheesecake, pipe on the mint Chantilly. Sprinkle over the grapefruit segments and finely grated lime. Decorate with mint leaves.