Graham crackers are the key to this easy and delicious no-bake cake.
Whisk together sugar, cornstarch, and 1/8 teaspoon salt in a medium saucepan. Whisk together milk and egg yolks in a bowl. Add milk mixture to the saucepan, along with butter. Cook, stirring, over medium until mixture comes to a boil. Let boil, stirring, 1 minute. Remove from heat and add vanilla.
Strain pastry cream through a fine-mesh sieve into a bowl. Cover with plastic wrap, pressing it directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until chilled, at least 2 hours or up to 2 days.
Whisk 1/2 cup heavy cream until firm peaks form. Whisk chilled pastry cream until smooth, then fold whipped cream into pastry cream.
Arrange 1/3 graham crackers on the bottom of a 9-by-13-inch pan. Spread half of the pastry cream mixture on top, smoothing it with a spatula to make an even layer. Repeat with half of remaining crackers and remainder of the pastry cream. Arrange remaining crackers on top of pastry cream.
Heat remaining 1 cup heavy cream in a small saucepan over medium-high until just boiling. Pour over chocolate in a small heatproof bowl; cover with a plate. Let stand until chocolate melts, then stir until smooth. Spread over graham cracker layer, smoothing top with an offset spatula. Refrigerate to set, at least 2 hours and up to overnight.