In a medium saucepan whisk together the sugar and cornstarch.

In a bowl whisk together the egg yolks and half and half. Turn the heat to medium and whisk the egg mixture into the sugar mixture.

Bring to just a boil while whisking constantly. Let bubble for 1 minute. Remove from heat. Whisk in the butter, lime zest and lime juice.

Set aside to cool for 10 minutes.

Meanwhile line an 8x8 square baking dish with plastic wrap.

Place your graham crackers down the bottom of the pan, I did 3 across and then broke apart some to fit the end of the pan.

Spoon 3/4 cup of the custard over crackers and spread over the crackers.

Repeat 3 times ending with a top layer of crackers.

Cover the top with plastic wrap and freeze for 8 hours or overnight.

Remove the cake from the pan and plastic wrap and place on a plate or platter and let thaw for 1 hour.

Meanwhile make the whipped topping by beating the heavy whipping cream and powdered sugar together with an electric mixture until you get soft peaks. Spread the whipped topping on the top of the cake and garnish with lime zest and lime slices if desired.