Adapted from Cook's Country
1 1/4 cups sugar
1/4 cup plus 2 tablespoons cornstarch
1/2 teaspoon salt
5 cups whole milk
1/4 cup (1/2 stick) butter, cut into pieces
5 teaspoons vanilla
2 packets Whip-It (whipped cream stablizer)
2 3/4 cups heavy cream
14 ounce box of graham crackers
1 cup semi-sweet chocolate chips
5 tablespoons light corn syrup
Whisk together sugar, cornstarch and salt in a large saucepan. Slowly whisk in milk, then heat to a boil, whisking, over medium heat. Lower heat and continue to whisk till mixture thickens and large bubbles appear. Remove from heat and add butter and vanilla. Put custard into a large bowl, cover with plastic wrap and cool in fridge for about 2 hours.
After about 2 hours, place one layer of graham crackers along bottom of 9 x 13 baking pan. Use smaller pieces to fill open spaces.
Whip 2 cups of cream with 2 packets of Whip-It till stiff peaks form. (Alternatively, you can use 1 1/4 teaspoons unflavored gelatin, softened in water, to stabilize whipped cream). Fold about 1/3 of whipped cream into cooled custard. When well combined, fold the rest of the whipped cream into custard.
Top graham crackers with half of the custard-whipped cream mixture. Layer with more graham crackers, then top with the rest of the filling. Place a 3rd layer of graham crackers over filling and refrigerate while making the chocolate topping.
Microwave chocolate chips, corn syrup and remaining 3/4 cup of cream for about 60 seconds. Whisk and microwave again till all chocolate is completely melted. Whisk mixture till smooth and allow to cool about 10 minutes. Pour over graham crackers, and using offset spatula, smooth to cover top of dessert. Chill at least 6 hours before serving.