01 -
In a medium saucepan, whisk together sugar, cornstarch, and 1/8 teaspoon salt. In a separate bowl, whisk milk and egg yolks. Pour the milk mixture into the saucepan with butter. Cook over medium heat, stirring until it boils. Continue boiling and stirring for 1 minute. Remove from heat and stir in vanilla.
02 -
Pour the pastry cream through a fine-mesh sieve into a bowl. Cover with plastic wrap pressed directly onto the surface to prevent skin formation. Refrigerate until cold, at least 2 hours or up to 2 days.
03 -
Beat 1/2 cup heavy cream until firm peaks form. Whisk the chilled pastry cream until smooth, then gently fold in the whipped cream.
04 -
Line a 9-by-13-inch pan with 1/3 of the graham crackers. Spread half of the pastry cream mixture evenly on top. Add another layer of crackers and remaining pastry cream. Top with final layer of crackers.
05 -
Bring remaining 1 cup heavy cream to a boil. Pour over chopped chocolate in a bowl and cover with a plate. Let stand until chocolate melts, then stir until smooth. Spread over the top layer of graham crackers.
06 -
Refrigerate cake until set, at least 2 hours or overnight before serving.