Step 1

Whisk together granulated sugar, cornstarch, and salt in a heavy saucepan. Whisk together egg yolks and half-and-half in a small bowl. Add to sugar mixture; whisk until smooth. Bring mixture just to a boil over medium, whisking constantly. Boil 1 more minute, whisking constantly; remove from heat. Add butter and zest; whisk until butter melts. Gradually whisk in juice just until blended. Fill a sink or large bowl halfway with ice. Pour custard mixture into a metal bowl; place bowl on ice. Let stand, stirring occasionally, until custard is cold and slightly thickened, 8 to 10 minutes.

Step 2

Line the bottom and sides of an 8-inch square pan with plastic wrap, allowing 4 inches to extend over all sides. Place 9 graham cracker squares in a single layer in bottom of pan to form a large square. (Crackers will not completely cover bottom.) Place a layer of banana slices, with sides touching, on graham crackers; place a layer of strawberry slices on top of bananas. Spread half of custard on top of strawberries. Repeat layers once, beginning and ending with graham crackers. Pull excess plastic wrap at sides tightly over cake; chill 4 hours or overnight.

Step 3

Lift cake from pan, using plastic wrap as handles.

Step 4

Beat cream with an electric mixer on high speed until foamy; gradually add powdered sugar, beating until medium-soft peaks form. Spread on top of cake. Top with crumbled graham crackers and strawberry halves.