Preheat oven to 350°F. Line a 9-inch square baking pan with foil, allowing 2 inches of overhang on either side.

To make the crust, in a large bowl, whisk together the graham cracker crumbs, sugar and salt. Add the butter and stir until evenly combined. Pour the mixture into the prepared pan and press out into an even layer. Bake until lightly golden brown, about 10 minutes. Remove from the oven and allow to cool while you make the filling.

Set a large bowl over a pot of simmering water, making sure that the water doesn’t touch the bottom of the bowl. Add the chocolate and butter and stir occasionally until melted through. Remove from the heat and stir in the sugar, vanilla and salt. Whisk in the eggs until smooth, then stir in the flour until just incorporated. Spread the batter evenly over the cooled crust. Bake for about 25 minutes, until the edge is set but the center is still slightly jiggly. Transfer to a wire rack and let cool completely.

To make the meringue place, in a medium heatproof bowl set over a saucepan with simmering water, whisk the egg whites with the sugar until the whites are warm and the sugar has dissolved, about 3 minutes. Transfer the egg whites to the bowl of a stand mixer fitted with the whisk attachment. Add the vanilla and cream of tartar and beat on medium speed until firm. Increase the speed to high and beat the meringue until stiff and glossy, about 5 to 7 minutes.

Mound the meringue on top of the cooled filling and swirl it around a bit. Use a kitchen torch or a preheat broiler to toast the meringue. Cut into bars and serve. Leftovers can be stored in the fridge for up to 2 days. Enjoy!