For the syrup, place all the syrup ingredients into a pan and bring to the boil, stirring occasionally with a wooden spoon. Boil for about 2-3 minutes, skimming the surface if necessary. Pass the syrup through a conical strainer and leave until completely cold before using. What you don't use for this recipe can be stored for several months.
For the blackcurrant purée
For the blackcurrant purée, place the blackcurrants into a blender with the syrup and blend until smooth. Pass the mixture through a conical strainer into a bowl.
For the Italian meringue
For the Italian meringue, place the sugar and water into a pan and slowly bring to the boil until the syrup reaches 110C/230F on a sugar thermometer.
At this stage, place the egg whites in the bowl of an electric beater and whisk until firm peaks form when the whisk is removed.
As soon as the sugar reaches 121C/250F, remove the pan from the heat. Immediately resume whisking the egg whites. With the beater at its lowest speed, pour in the melted sugar, in a steady stream, making sure it is clear of the beaters. Continue beating for about five minutes, until the meringue is tepid.
For the blackcurrant mousse
For the blackcurrant mousse, soak the gelatine leaves in cold water for 15 minutes, then drain.
Reserve four tablespoons of the blackcurrant purée and place the rest into a pan. Add the powdered milk and vanilla pod and bring to the boil.
Place the egg yolks and sugar into a bowl and whisk until the mixture forms a ribbon when the whisk is removed. Pour the blackcurrant mixture onto the egg mixture, whisking continuously, then pour this custard mixture back into the pan. Heat gently, stirring continuously with a spatula, until the custard is thick enough to coat the spatula. Do not allow it to boil.
Take the pan off the heat and stir the drained gelatine into the custard. Discard the vanilla pod, then pass the custard through a conical strainer into a bowl and leave to cool, stirring occasionally to prevent a skin forming.
For the base
For the base, place the sponge flan base onto a board. Mix together 50ml/2fl oz of the syrup and one tablespoon of the reserved blackcurrant purée and brush this mixture over the sponge. Place a 25cm/10in flan ring (6cm/2½in deep) onto the sponge and cut out a circle to form the base.
When the custard is lukewarm, gently fold in half of the Italian meringue - you can use the rest in another recipe. Stir in the crème de cassis, then fold in the whipped cream. You must now assemble the dessert immediately, before the gelatine causes the mixture to set.
Fill the flan ring with the blackcurrant mousse mixture, leaving a small gap at the top, and carefully smooth over the surface with a palette knife. Place in the freezer or a very cold fridge for several hours.
For the glaze
For the glaze, soak the gelatine leaves in cold water for 15 minutes, then drain well. Place 50ml/2fl oz of the syrup and the remaining three tablespoons of blackcurrant purée into a pan and heat gently, then remove from the heat and stir in the gelatine. Leave to cool, then spread the glaze over the top of the mousse. Place into the fridge to chill before serving.
To serve, remove the tart ring by heating the outside of the ring with a blowtorch for a few seconds, then slide the ring upwards, rotating it slightly.