Arrange the mango slices in an even layer in a shallow, heatproof dish. Put the cream and fromage frais in a saucepan and heat gently until bubbles begin to appear. Put the egg yolks, caster sugar, vanilla and cornflour in a large bowl. Stir with a wooden spoon until you have a smooth mixture. Very carefully pour the hot cream and fromage frais into the bowl and stir with a wooden spoon to blend everything together. Pour the mixture back into the saucepan and place it over a low heat. Bring the mixture to the boil, stirring it regularly. Remove the saucepan form the heat and leave to cool for 10 minutes. Stir well then pour over the mango slices in the dish. Leave until cold then chill in the refigerator for 30 minutes. Sprinkle demerara sugar over the top in an even layer. Put under the grill until the sugar melts. Cool. Chill for 15 minutes.