In a very clean and dry bowl of a stand mixer, using the whisk attachment, whip the egg whites on low for a few minutes until frothy, then increase the speed to medium high and whip until soft peaks form.
Add the sugar very gradually, shaking in with the mixer on. Add the cream of tartar then continue to whip on medium high or high, until stiff and glossy peaks form.
Fold the vanilla in using a spatula. Spoon the meringue into a pastry bag fit with a star tip.
When the chocolate filling’s twenty minute timer has gone off, remove the tart from the oven.
Pipe the meringue over the top of the chocolate pie filling, which will still be soft, making pointed circular decorations across the top. Or if you prefer, use a spatula to spread meringue over the top of the filling, making some peaks in the meringue, while also being very careful as the filling will still will be soft.
Bake the tart with the meringue for 18 - 25 minutes, and until peaks are getting golden. Remove from the oven and cool. Refrigerate after cooled. When ready to serve, remove from the refrigerator and remove from the tart pan or spring form pan and parchment Serve chilled. Makes 6-8 slices.