Peel and core the apples and cut into small pieces. Drizzle with roughly a teaspoon of lemon juice.
In a pot, combine the apple juice, custard powder, and 1 packet of vanilla sugar. Heat over medium heat, stirring constantly for about 5 minutes until thickened and steaming. Fold in the apple pieces. If desired, add liquid sweetener to taste. Remove from heat and let cool, stirring occasionally for about 10-15 minutes. Divide into bowls.
In a bowl, whip the mascarpone with the cream, roughly a teaspoon of lemon juice, and 1 packet of vanilla sugar until creamy, about 2-3 minutes. Spoon onto the cooled apples in the bowls. Garnish as desired.
Refrigerate for about 2 hours, or until chilled and ready to serve.