Instructions

Combine 1¼ cups (8¾ ounces/248 grams) sugar, 6 tablespoons (1½ ounces/43 grams) cornstarch, and 1 teaspoon salt in large saucepan. Whisk 5 cups whole milk into sugar mixture until smooth.
Bring mixture 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.
Remove from the heat and whisk in 4 tablespoons unsalted butter (cut into 4 pieces) and 5 teaspoons vanilla extract.
Strain through fine-mesh strainer set over clean bowl. Place piece of greased parchment directly on surface of pudding. Refrigerate until cool, about 2 hours.
Stir 2 tablespoons water and 1¼ teaspoons of unflavored 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.
Whisk one-third whipped cream into chilled pudding, then gently fold in remaining whipped cream, in two additions, 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 5½ cups) and another layer of graham crackers.
Repeat with remaining pudding-whipped cream mixture and remaining graham crackers.
Combine 1 cup (6 ounces/170 grams) semisweet chocolate chips, ¾ cup heavy cream, and 5 tablespoons light corn syrup in bowl. 
Microwave, stirring occasionally, until smooth, 1 to 2 minutes. Cool glaze to room temperature, about 10 minutes.
Cover graham crackers with glaze.
Refrigerate cake for 6 to 24 hours.
Serve chilled. (Éclair cake can be refrigerated for up to 2 days.)