Carefully fold the dry ingredients, in two batches, into the beaten egg whites with a flexible rubber spatula. When the mixture is just smooth and there are no streaks of egg white, stop folding and scrape the batter into the pastry bag (standing the bag in a tall glass helps if you’re alone).
To make the prune filling:
To make the chocolate filling:
When the cream just begins to boil at the edges, remove from heat and add the chopped chocolate. Let sit one minute, then stir until smooth. Stir in the pieces of butter. Let cool completely before using.