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Line a 9 x 9 inch square pan with parchment paper. I like to make the sides of the parchment paper a bit higher than the pan so that when the cake is done, you can lift up the whole thing using the excess parchment as handles, which will make it easier to slice and serve.
Add condensed milk and lime juice to a large mixing bowl. Whisk until evenly combined. Let the mixture sit out for 5-10 minutes to thicken.
While it is thickening, add the heavy cream to a bowl of a stand mixer. Beat at highest speed until stiff peaks form. Once your lime juice mixture is thick (it should be like pudding or custard), add in the whipped cream and lime zest. Gently fold them in with a spatula until evenly incorporated.
Spread a very thin layer of lime cream across the bottom of the lined pan. Line the bottom with a layer of graham crackers. You should be able to fit 3 rows with 7 crackers in each row. Graham crackers come in a sheet with 4 crackers. I recommend cutting the rectangle sheet in half so that it becomes 2 squares (with 2 crackers on each square). I had trouble breaking them precisely in half with my hands so I found it easier to use a knife. Place 3 of those squares and then a single cracker for each row. I like keeping 2 together instead of just 7 individual ones because there will be less breaks/gaps in the cake layers when you slice. (See photo in post for reference)
Add a thin layer of cream over the layer of graham crackers. Then add a second layer of graham crackers over the first. So each cake layer will have a double layer of graham crackers. This creates a thicker cake layer.
Add a thick layer lime cream on top of the crackers (I used a little over 2 cups), spreading it evenly across. You want the cream layer to be about ½ inch thick. I like using an offset spatula to help me spread it evenly across but a regular spatula should also work. Add another double layer of crackers (using same method as first double layer), followed by another thick layer of cream. Add one more double layer of crackers.
Top the last layer of crackers with the remaining lime cream. It will be a thinner layer but you should have enough to cover the surface. Cover the pan with plastic wrap and refrigerate the cake for at least 4 hours or overnight to allow the flavors to meld and the cookies to soften. Keep cake in fridge until ready to serve.
On the day of serving, you can also decorate the top of the cake with optional frosting and/or fresh lime slices and zest. Cake is best served chilled.
Optional Frosting: Add 1.5 cups heavy cream and 4.5 tbsp sugar to a large mixing bowl of a stand mixer. Beat at highest speed until medium peaks form. Frost over key lime cake (you don't need to use up all the frosting but this recipe makes enough for a very generous thick layer across). You can also make more of the lime cream filling for an optional frosting as well.
You will need 1 (14 oz) can of condensed milk. 14 oz refers to weight, not volume. This recipe will not work dairy free condensed milk substitutes. The recipe relies on the reaction of the milk protein in the condensed milk with the acid of the lime juice.
We used the juice of fresh key limes. You can substitute with regular limes. You can also use bottled lime juice, just make sure the only ingredient is lime juice (no added water, etc). I’ve made this cake successfully with Santa Cruz Organic 100% lime juice and Whole Foods 365 Organic Lime Juice.
This is a very rich cake so we recommend small slices.