Preheat oven to 350 F and lightly grease a 9 x 13 inch glass baking dish.
Mix everything together thoroughly in a bowl. Set aside.
Combine the butter, sugar and vanilla in a medium bowl until combined. Beat in the eggs, one at a time until everything is incorporated.
In another bowl, sift together the salt, cocoa and flour. In two additions, mix the dry into the wet and stir until combined. Stir in the dark chocolate shards.
Pour the brownie mixture into the prepared dish and smooth it out so that it’s as flat as you can make it. Top it off with the buttertart layer.
Bake in the preheated oven for about 40-45 minutes. Don’t bother with the toothpick test- the gooey buttertart layer will always be gooey and will give a false impression of doneness. Check the bottom of the pan for doneness and press the top to ensure it no longer giggles and is dry.
Allow to cool for a few hours until room temperature before slicing, dusting with icing sugar (if desired) and serving.