Place the jelly crystals in a heatproof jug and pour over 285ml boiling water. Stir until the jelly crystals have dissolved. Add 285ml cold water and stir, then set aside and leave to cool.
Meanwhile, cut each mini roll into 6 thin slices. Place 6 slices in the base of four glass dishes or a small glass serving bowl.
Divide the raspberries between the four dishes or serving bowl, placing them between and on top of the mini roll slices.
When the jelly has cooled, divide it between the four dishes, or pour into the serving bowl over the mini roll slices and raspberries. Place in the fridge to set firmly for a minimum of 2 hours.
Purée the raspberries in a food processor or with a stick blender. Place the raspberry purée in a sieve placed over a small bowl, pushing the purée through with the back of a spoon to remove the seeds. Scrape all of the purée off the underneath of the sieve into the bowl and discard the seeds.
Put the egg yolk, cornflour and sweetener or sugar in a heatproof jug. Add 1 tbsp of the milk and stir until smooth.
Pour the remaining milk into a small saucepan and place over a medium heat. Heat until steaming hot, taking care not to let it burn or boil over.
Gradually pour the steaming hot milk into the mixture in the jug, stirring with a wooden spoon or whisking with a balloon whisk until completely blended.
Pour the mixture back into the saucepan and place over a medium heat. Cook the custard, stirring constantly with a wooden spoon or balloon whisk, for about 3-4 minutes, until there are a few small bubbles starting to appear on the surface and the custard has thickened.
As soon as the custard has thickened, remove it from the heat. Don't let the custard overheat or boil otherwise it may burn or split. Stir in the raspberry puree, vanilla extract and a few drops of pink food colouring (optional). Scrape the custard in a small bowl and cover with a disc of non-stick baking paper, to stop a skin forming on the top. Set aside and leave to cool.
When the custard is just cool, remove the paper disc and stir with a balloon whisk to ensure it's smooth. Divide the custard between the four dishes or pour into the medium serving bowl, spreading out over the top of the set jelly. Place in the fridge to chill for a minimum of 30 minutes.
When ready to serve, decorate with a swirl of reduced-fat aerosol cream and a raspberry (optional).