Step 1

Combine sugar, cornstarch, and salt in large saucepan. Whisk milk into sugar mixture until smooth and bring to boil, scraping bottom with rubber spatula, over medium-high heat. Immediately reduce heat to medium-low and cook, continuing to scrape bottom, until thickened and large bubbles appear on surface, 4 to 6 minutes. Off heat, whisk in butter and vanilla. Transfer pudding to large bowl and place plastic wrap directly on surface of pudding. Refrigerate until cool, about 2 hours.

Step 2

Stir water and gelatin together in bowl and let sit until gelatin softens, about 5 minutes. Microwave until mixture is bubbling around edges and gelatin dissolves, 15 to 30 seconds. Using stand mixer fitted with whisk, whip 2 cups cream on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until soft peaks form, about 2 minutes. Add gelatin mixture and whip until stiff peaks form, about 1 minute.

Step 3

Whisk one-third whipped cream into chilled pudding, then gently fold in remaining whipped cream, 1 scoop at a time, until combined. Cover bottom of 13 by 9-inch baking dish with layer of graham crackers, breaking crackers as necessary to line bottom of pan. Top with half of pudding-whipped cream mixture (about 5 1/2 cups) and another layer of graham crackers. Repeat with remaining pudding-whipped cream mixture and remaining graham crackers.

Step 4

Microwave chocolate chips, remaining 3/4 cup cream, and corn syrup in bowl, stirring occasionally, until smooth, 1 to 2 minutes. Cool glaze to room temperature, about 10 minutes. Cover graham crackers with glaze and refrigerate cake for 6 to 24 hours. Serve. (Éclair cake can be refrigerated for up to 2 days.)