To make the crust

Preheat the oven to 350° F.

Line a 9-inch-square baking pan with foil, allowing 2 inches of overhang on two sides. (I also spray the foil to make sure nothing sticks.)

In a medium bowl, using a fork, mix all of the ingredients until evenly moistened.

Press the crumbs evenly into the bottom of the prepared pan. Bake for 8 to 10 minutes, just until lightly browned. Let cool completely. Leave the oven on.

To make the brownie layer

While the crust cools, in a heatproof medium bowl set over a saucepan of simmering water, melt the butter with the chocolate over moderate heat, stirring occasionally, until smooth, about 5 minutes.

Remove from the heat and whisk 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 rack and let cool completely.

To make the marshmallow meringue

Preheat the broiler.

In a heatproof medium bowl set over a saucepan of simmering water, whisk the egg whites with the sugar until the whites are warm and the sugar is dissolved, about 3 minutes.

Transfer the egg whites to the bowl of a stand mixer fitted with the whisk. Add the vanilla and cream of tartar and beat at medium speed until firm. Increase the speed to high and beat the meringue until stiff and glossy, 5 to 7 minutes.

Mound the meringue on top of the filling, swirling it decoratively. Broil the meringue 8 inches from the heat until lightly browned at the tips, about 1 minute. (Watch carefully, the meringue will brown very quickly.) Cut into bars and serve.

The bars can be refrigerated for up to 2 days (if they last that long).