Soak the gelatine in cold water. Season the buttermilk with the lemon zest. Melt both types of sugar with 4 tablespoons of lemon juice in a small saucepan over medium heat and remove from the stove.
Squeeze out the gelatine well and melt it in the hot sugar syrup. Gradually stir in the buttermilk, then transfer to a bowl. Chill for about 30 minutes, stirring occasionally, until the buttermilk begins to gel.
Whip the whipped cream until it is half stiff. First carefully fold half of the cream into the buttermilk cream with a whisk, then the other half. Pour into a serving bowl and refrigerate for at least 3 hours.
Clean the raspberries. Finely puree a third of the berries in a tall container together with the powdered sugar and 1 tablespoon lemon juice using a hand blender. Add the remaining berries and mix.
Peel the white chocolate from the bar with the peeler to make curls. Cut off the dumplings from the buttermilk mousse and arrange on plates. Serve garnished with the raspberries and chocolate.