Step 1
Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with cooking spray and line with parchment paper.
Step 2
Mix cake mix 1 cup water eggs and vegetable oil in a stand mixer on low speed for 30 seconds. Increase speed to medium and mix for 2 minutes more. Pour batter into the prepared baking dish.
Step 3
Bake in the preheated oven until cake is golden brown about 25 minutes. Cool cake at room temperature for 10 minutes. Keep oven on.
Step 4
Line a baking sheet with parchment paper.
Step 5
Mix graham cracker crumbs and melted butter together in a bowl; spread onto the prepared baking sheet.
Step 6
Bake graham cracker crumb mixture for 10 minutes. Cool to room temperature and store in an airtight container.
Step 7
Mix strawberries blackberries raspberries sugar 1/3 cup water and 2 tablespoons lemon juice together in a saucepan; bring to a boil. Reduce heat and simmer stirring occasionally until fruit is soft and easily mashed 7 to 8 minutes. Mash berries together with a fork and cool for 5 minutes.
Step 8
Poke 5 rows of 6 holes in the cake using the handle of a wooden spoon creating 30 holes. Drizzle 1/3 cup sweetened condensed milk over the entire cake. Pour berry mixture over cake and spread using a spatula ensuring berry mixture goes into the holes. Cover cake and refrigerate at least 1 hour.
Step 9
Place cream cheese in the bowl of a stand mixer fitted with a whisk attachment; mix on high until very smooth at least 3 minutes. Add the remaining sweetened condensed milk 1/4 cup lemon juice and sea salt; mix for 1 minute. Spread cream cheese mixture over the cooled cake. Cover and chill cake 3 hours to overnight.
Step 9
Slice into 12 large pieces and top each with graham cracker crumb mixture.