For the cake

1. Line a 9x15-inch loaf pan with plastic wrap, allowing to hang over all the sides.
2. In a large bowl using a hand-held mixer or stand mixer with the paddle attachment, beat the mascarpone cheese, powdered sugar, and lemon zest on medium until fluffy.  Reduce the speed to low and slowly stream in the heavy cream.  Increase mix to medium, and continue to mix until medium peaks form.
3. Place a layer of graham crackers on the bottom of your prepared loaf pan, trimming with a knife as necessary.  Spread 1/3 of the lemon cream over the graham crackers and top with 1/3 of the mascarpone filling.  Spread evenly.  Top with another layer of graham crackers and repeat with the lemon cream and mascarpone filling.  Repeat one more time so that you have three layers of graham crackers, lemon cream, and mascarpone filling.  Top with a final layer of graham crackers.
4. Cover with plastic wrap and freeze for at least 6 hours.
5. Uncover the cake and invert onto a cake stand or serving dish.  Unmold and remove the plastic wrap.

For the whipped topping

1. In a large bowl using a hand-held mixer or stand mixer with the whisk attachment, whisk the heavy cream, powdered sugar, and vanilla on medium-low speed until mixture becomes frothy.  Increase speed to medium-high and continue whisking until medium peaks form about 3 minutes.  Careful not to over mix or else it will become thick and lumpy.
2. Using an offset spatula, cover the cake with the whipped cream.
3. Serve immediately or store in the freezer until ready to serve.