Frozen Avocado Cake

Preheat the oven to 350°F (180°C). Lightly butter the inside of a 9-inch (23-cm) springform pan.
Make the crust: Crumble the crackers and cookies into a medium bowl. Use a pestle or the bottom of a heavy bottle to crush them into fine crumbs. Add ⅓ cup (75 ml) of the condensed milk and stir until very well combined. (You can grind the crumbled graham crackers and mix them with the condensed milk in a food processor if you prefer.) Press the crumb mixture firmly and evenly into the bottom of the pan. Bake until the crust looks a little darker and smells sweet and toasty, 12 to 15 minutes. Cool the crust in the pan on a wire rack.
One at a time, cut an avocado in half lengthwise. Twist the halves to separate them. Hold the half with the pit in one hand. Holding the knife in your other hand, rap the knife blade into the pit to lodge it there. Twist the knife to loosen and remove the pit. Use a large spoon to scoop the avocado flesh onto a chopping board, discarding the skins. Coarsely chop the avocados.
Add the chopped avocados, cream cheese, remaining condensed milk, and salt to a food processor. (You can combine the filling in a blender instead but you may need to work in batches.) Grate the zest of 1 lemon and set aside. Squeeze the zested lemon and the limes (you should have about ⅓ cup/75 ml lemon), reserving the remaining lemon for garnish. Add the juice to the avocados and process the mixture until smooth, scraping down the sides of the bowl as needed.
Spread the filling in the cooled crust and sprinkle with the grated lemon zest. Cover the pan and freeze until the filling is firm, at least 6 hours or preferably overnight.
Let the frozen cake stand at room temperature for 15 to 30 minutes before slicing. Dipping a thin knife into hot water between each cut, slice the cake into wedges and serve, topping each serving with freshly grated lemon zest.