Heat the oven to 350°F with the rack in the middle position. Mist an 8-inch square baking pan with cooking spray (use a broiler-safe pan if you will be toasting the meringue under the broiler). Line the pan with a 14-inch length of foil, folded widthwise so it fits neatly in the bottom of the pan; allow the excess to overhang the sides. Mist the foil with cooking spray.

To make the crust, in a medium bowl, stir together the cracker crumbs, sugar and salt, breaking up any lumps of sugar. Drizzle in the melted butter, then stir until the crumbs are evenly moistened. Transfer the mixture to the prepared pan and press it firmly into an even layer. Bake until light golden brown, 10 to 12 minutes. Cool completely on a wire rack; leave the oven on.

To make the brownie layer, in a medium saucepan over medium, bring 1 inch of water to a bare simmer. Put the butter and chocolate into a heatproof medium bowl and set the bowl on top of the saucepan; make sure the bottom does not touch the water. Stir often until the mixture is completely smooth. Remove the bowl from the pan; reserve the saucepan and water. To the butter-chocolate mixture, add the sugar, salt and vanilla. Whisk until well combined, about 1 minute; the mixture will be grainy. Add the eggs and whisk gently until incorporated and the mixture is smooth and shiny, about 1 minute. Add the flour and fold with the spatula until no streaks remain.

Pour the batter onto the cooled crust. Bake until a toothpick inserted at the center of the brownies comes out with a few moist crumbs clinging to it, 20 to 22 minutes. Cool completely in the pan on a wire rack, about 1 hour.

To make the meringue, in the reserved saucepan, once again bring about an inch of water to a bare simmer. In the bowl of a stand mixer or, if using a hand mixer, in a large heatproof bowl, combine the egg whites, sugar and cream of tartar. Set the bowl on the saucepan; make sure the bottom does not touch the water. Whisk constantly until the sugar has dissolved and the mixture is just warm to the touch, 3 to 4 minutes; it will be frothy and milky in appearance.

Remove the bowl from the pan and attach it to the stand mixer stand along with the whisk attachment, or have ready a hand mixer. Add the vanilla and begin beating on low; gradually increase the speed to high over the next 1 to 2 minutes, then whip until the meringue is glossy and holds stiff peaks, 5 to 7 minutes total; do not overbeat.

If using a broiler to toast the meringue, heat the broiler with a rack positioned about 10 inches from the element. Pile the meringue onto the brownies (still in the pan) and spread it to cover the surface, then use the back of a spoon to create swirls in the meringue. If using a kitchen torch, ignite the torch and toast the meringue until caramelized in spots. If broiling, slide the pan into the oven and broil, keeping the oven door ajar and watching carefully to prevent scorching, until the meringue is toasted, 1 to 1½ minutes; rotate the pan halfway through.

Let stand for about 30 minutes. Remove the bars from the pan, using the foil overhang as handles, and set on a cutting board. Using a chef’s knife, cut into 16 squares, wiping the knife blade after each cut for the cleanest slices.