In a small sauce pot, combine the blueberries, lemon juice, sugar, cornstarch and salt, reserving the lemon zest for later use. Set the pot over medium-high heat and bring to a boil. Simmer for 4-5 minutes to allow the berries to burst open, stirring regularly. Then cool completely. (You can place the pot in the refrigerator or freezer to speed this up.)
Meanwhile, lay two long pieces of plastic wrap out on the counter, one overlapping the edge of the other, to create a large square. Line an 8x8-inch baking dish with the plastic wrap, letting the extra plastic hang over the edges.
For the Whipping Cream:
Pour the heavy cream in the bowl of an electric mixer. Add the sugar and vanilla. Use a whip attachment to whip the cream into firm peaks. Scrape the bowl and add the lemon zest. Whip again to make sure the whipped cream is thick and smooth.
Once the blueberry filling is no warmer than room temperature, start layering the cake. Spread 1/2-inch whipped cream over the bottom of the pan. Carefully spread 1/3 of the blueberries over the cream. Then press nine square graham crackers evenly into the cream. Repeat in this order two more times, creating 3 layers each of cream, blueberries, and graham crackers.
Wrap the excess plastic up over the top of the cake. Let it sit on the counter for 15 minutes, so the moisture can soften the graham crackers. Then freeze for 2 hours, or until solid. Keep the remaining whipped cream in the refrigerator for later use.
Once frozen, unwrap the top of the cake and flip it out onto a flat platter. Peel off the plastic wrap. Spread the rest of the whipped cream evenly over the top and sides of the cake. Crumble the last three graham crackers and sprinkle over the cake. Then freeze to harden. After the outer layer is hard, either cut and serve, or wrap well so it does not get freezer burn.
Don't like blueberries? You can use this exact same recipe substituting strawberries, cherries, or raspberries. If well wrapped, this cake can last in the freezer for months.