These bars, while not a snap to make, are totally worth your time — especially if you like Mallomar cookies, which they're meant to emulate. A buttery shortbread crust is topped with homemade marshmallow, then spread with dark chocolate ganache. Who could ask for anything more?
To make the crust: Preheat the oven to 300°F. Lightly grease a 9" x 13" pan, and line it with parchment, if desired; this will help you get the bars out of the pan intact.
Beat together the butter, sugar, and vanilla, then beat in the flour. The mixture may seem a little dry at first; keep beating until it comes together. If it absolutely won't come together, dribble in up to 1 tablespoon of water, until it does. This is a stiff dough.
Press the dough into the prepared pan, smoothing the surface with your fingers. Use a fork to prick the dough all over; this allows any steam to escape, and prevents the shortbread from bubbling as it bakes.
Bake the shortbread until it's a light golden brown across the top surface, and a deeper golden brown around the edges, about 35 minutes. Remove it from the oven, and set it aside while you make the filling.
To make the marshmallow filling: Combine the gelatin and 1/2 cup cool water in the bowl of an electric mixer fitted with the whisk attachment. Stir to make sure all the gelatin is moistened; it will turn into a soft block.
Combine the sugar, corn syrup, salt, and 1/2 cup cool water in a small, deep saucepan. Cook the mixture over medium heat, stirring, until the sugar dissolves. Raise the heat to high and cook, without stirring, until the syrup reaches 240°F on a candy thermometer. Remove from the heat.
With the mixer on low speed, slowly pour the sugar syrup into the softened gelatin. Increase the speed to high, and whip until the mixture is very thick and fluffy, and has cooled to lukewarm, 3 to 10 minutes (depending on the mixer and attachment you use; a stand mixer using the whisk attachment will work more quickly than a hand mixer equipped with beaters). The mixture should be cool enough that you can spread it into the pan without burning your fingers, about 95°F. Add the vanilla towards the end of the mixing time.
Spread the marshmallow mixture atop the crust, using your wet hands to smooth and flatten the marshmallow as best you can. You have to do this quickly, as the marshmallow will begin to set. Let the marshmallow cool to room temperature before glazing the bars.
To make the glaze: Combine the chocolate and cream in a saucepan or microwave-safe bowl. Heat until the cream starts to steam, and develops small bubbles around the edge. Remove from the heat (or microwave), and stir until smooth, reheating briefly if necessary to completely melt the chocolate.
Spread the chocolate atop the marshmallow. Let cool and set at room temperature before cutting into bars.
