Combine sugar, cornstarch and salt in a large saucepan. Whisk milk in until smooth and bring to a boil, scraping the bottom with a rubber spatula over medium high heat. Immediately reduce heat to medium low and cook, continuing to scrape bottom, until thick and large bubbles appear on the surface, 4-6 minutes. Off heat, whisk in butter and vanilla. Transfer pudding to a large bowl and place plastic wrap directly on the surface. Refrigerate until cool, about 2 hours or overnight.
Stir water and gelatin together in a bowl and let sit until gelatin softens, about 5 minutes. Microwave until mixture is bubbling around the edges and gelatin dissolves, 15-30 seconds. Using a stand mixer fitted with a whisk, whip 2 cups cream on medium low speed until foamy, about a minute. Increase speed to high and whip until soft peaks form, about 2 minutes. Add gelatin mixture and whip until stiff peaks form, about a minute more.
Whisk one-third whipped cream into chilled pudding, then gently fold in remaining whipped cream, one scoop at a time until combined. Cover the bottom of a 13 X 9 inch baking dish with a layer of graham crackers, breaking the crackers as necessary to line the bottom of the pan. Top with half of the pudding mixture and another layer of graham crackers. Repeat with remaining pudding mixture and remaining graham crackers.
Microwave chocolate chips, remaining 3/4 cup cream and corn syrup in a bowl, stirring occasionally until smooth, 1-2 minutes. Cool glaze to room temp, about 10 minutes.Cover graham crackers with glaze and refrigerate cake for 6 - 24 hours.