Éclair Cake

Place about 7 of the graham cracker sheets in an even layer in a 9- by 13- inch baking dish, breaking up crackers as needed to fill space; set aside.

In a large bowl, whisk together sugar, vanilla, and 1 1/2 cups of the heavy cream until stiff peaks form, about 5 minutes. (See notes.) Place pastry cream in a large bowl, and, using a rubber spatula, gently fold whipped cream into pastry cream until just combined. Using an offset spatula, spread half of the pastry cream mixture (about 18 ounces; 510g) in an even layer over graham crackers. Place about 8 of the graham cracker sheets in an even layer over pastry cream mixture, breaking up crackers as needed to fill space. Using the offset spatula, top with remaining half of pastry cream mixture in an even layer. Place remaining 9 graham cracker sheets in an even layer, breaking up crackers as needed to fill space. Cover with plastic wrap and refrigerate until pudding is set, about 1 hour.

Place chocolate in a medium heatproof bowl; set aside. In a small saucepan set over medium-high heat, bring butter and remaining 1 cup cream to a gentle simmer, stirring constantly, until it begins to bubble around the edges, about 4 minutes. Pour over chocolate (do not stir); let stand for 10 minutes. Using a rubber spatula, gently stir together until smooth, about 1 minute. Remove cake from refrigerator. Pour chocolate mixture over top of set cake, using an offset spatula to spread ganache into an even layer. Re-cover with plastic wrap, making sure it doesn't touch the surface of the chocolate, and refrigerate until set and graham crackers are softened, at least 2 hours or up to 12 hours.