Mallomars were introduced in 1913 and have since remained to be a New York City favorite. In fact, 85% of Mallomars are sold in the New York metropolitan area, but only from October through April. It's said to be because they easily melt in the summer heat, but most consumers believe it to be a marketing strategy to ensure greater sales figures by limiting their availability. Most just stock up when the cookies make their annual return to stores, and they just refrigerate them in the summer. But why wait for October when you can make your own Mallomars anytime?
In large bowl, stir graham crumbs and butter until crumbs are evenly moistened and mixture is firm (i.e., holds its shape when pressed into place). Press firmly into bottom of very lightly greased 9x13-inch baking pan. Optional: Bake at 350°F for 10 minutes; let cool.
(Baking the base crust is optional but results in a firmer base that won't crumble as easily when cutting into bars.)
Place 1/4 cup water in small bowl and sprinkle gelatin over top. Let sit at least 5 minutes to soften.
Stir 1/4 cup corn syrup, remaining 1/4 cup water, and sugar in medium saucepan over medium-high heat until dissolved. Continue to cook, without stirring, until mixture registers 240°F on an instant read thermometer.
Immediately pour hot syrup into bowl of standing mixer. Add remaining 1/4 cup corn syrup, softened gelatin, and vanilla. Turn mixer to medium speed and beat until combined, about 1 minute. Increase speed to medium high and beat until light and fluffy, about 8 minutes.
(Note: avoid over-whipping the marshmallow mixture or it will cool down, making it difficult to spread smoothly.)
Pour marshmallow mixture over graham cracker base and smooth top with a spatula. Chill until set, about 30 minutes.
Place chocolate in medium bowl; set aside. In a small saucepan, heat cream until simmering. Pour hot cream over chocolate, and whisk until melted and smooth. Pour melted chocolate mixture over the marshmallow layer, and chill until chocolate is set, about 15 minutes.
Once set, use a hot, wet knife to cut 4 rows vertically and 6 horizontally, to create 24 squares.