Set your oven to 350°F. Combine the 6 crushed graham crackers, chopped almonds, brown sugar, salt, and melted butter in a food processor and pulse until the mixture looks like wet sand with some texture still visible—it should hold together without turning into a paste. Transfer the mixture to a 9x13 inch baking dish and press it down firmly and evenly using the bottom of a measuring cup. Bake for 10 minutes until the crust turns lightly golden, then remove from the oven and allow it to cool completely to room temperature, approximately 10-15 minutes.

Once the crust has cooled, take about 1.5 pints of the softened vanilla ice cream and spread it evenly across the crust—an offset spatula dipped in warm water makes this much easier. Stir the lemon zest into the blackberry jam to add brightness, then spread this jam mixture in an even layer over the ice cream. Break the remaining 3 graham crackers into crumbs by hand and scatter them over the jam layer, then spread the remaining 1.5 pints of softened ice cream on top, smoothing it into an even surface.

Place the assembled cake in the freezer for at least 2 hours until the ice cream becomes completely firm throughout. Once the cake is frozen solid, spread the whipped topping evenly over the top using an offset spatula or the back of a spoon to create a smooth, even layer that will support the berries.

Create a flag pattern by arranging the raspberries and blackberries on the whipped topping in horizontal stripes. For a classic American flag appearance, make two raspberry stripes on the bottom, one blackberry stripe in the middle, and finish with a raspberry stripe on top—or adjust the pattern to your liking. Store the finished cake in the freezer until you're ready to serve, then slice and plate immediately to prevent melting.