To make pudding: In a large heat-safe bowl, whisk together corn starch, sugar, cake mix, salt and milk until combined. Place bowl over a double boiler with water simmering below, and cook for 30-35 minutes, stirring occasionally, until thickened and can coat the back of spoon and drizzle ribbons. Pour through a fine mesh strainer to remove any lumps. Let cool completely.
For cake: Begin by making the cake batter pudding so it has ample time to cool. It’s great if you can make it ahead of time.
Line a 9×5 loaf [you can actually use any other pan that works for you] pan with plastic wrap that overlaps the edges and is long enough to help lift the cake from the pan.
In the bowl of your electric mixer, add cold heavy cream and beat on low speed until stiff peaks form. This will take a while! Add in sugar and vanilla and beat until just combined. Remove 1 1/2 cups whipped cream from the large bowl and place in a smaller bowl. Place both bowls in the fridge until everything is ready to be assembled.
Spread a layer (about 1/2 inch thick) of whipped cream all along the bottom and sides of the pan. Place a layer of graham crackers on top (as many as you can fit), spread with whipped cream, top with sprinkles, then place another layer of grahams. Spread a layer (again, about 1/2 inch thick) of pudding over the grahams, then place another layer of grahams. Repeat until you reach the top, making sure to push whipped cream and pudding down the open sides, then end with a layer of whipped cream. Place in the freezer, freezing at least 4-6 hours, or overnight. Place remaining whipped cream in the freezer.
Remove from freezer and lift the cake out, flipping over onto a small pan or cutting board that will fit in the freezer. Spread whipped cream all over cake for a finishing presentation, then cover in sprinkles. Freeze for at least another 2 hours, or as long as needed. When ready, cut and serve immediately. Additionally you can just scoop it out of the pan and serve it in bowls. Keep in the freezer.