Prepare the vanilla custard pudding with the pudding powder, 400 ml milk, and 3 tablespoons of sugar following the package instructions, using medium heat. Allow to cool to room temperature, approximately 20-30 minutes.
In a large bowl, whisk together the cooled pudding, 500 g mascarpone, and 6 cl amaretto until smooth. Season to taste with vanilla sugar. Cut the 15 amarena cherries into small pieces and stir into the cream. If desired, stir in some of the amarena juice from the jar. Roughly crumble the 150 g amarettini biscuits.
In a large bowl or in four individual dessert bowls, layer the cream mixture alternately with the crumbled amarettini biscuits, starting with cream and ending with a layer of cream on top. Refrigerate for at least 1 hour. Just before serving, decorate with whole amarettini biscuits and whole amarena cherries.