Combine the pudding mixes and milk in a large bowl and whisk for 2 minutes, until smooth and beginning to thicken. Refrigerate for 5 minutes.
2 (3.4-ounce) boxes instant vanilla pudding, 3 1/2 cups cold milk
Beat heavy cream and powdered sugar in a separate bowl until stiff peaks form. Fold whipped cream into the vanilla pudding, stirring until completely incorporated.
1 cup heavy cream, 1/4 cup powdered sugar
Arrange graham crackers in the bottom of a 9x13-inch dish in an even layer, breaking them up as needed to fit the pan. Spread 1/2 of the pudding mixture over the bottom layer of graham crackers. Arrange additional graham crackers in another even layer on top of the pudding, then repeat with the remaining pudding mixture and a final layer of graham crackers.
3 sleeves graham crackers
In a microwave-safe bowl, heat the butter and milk until the butter has melted, then add the powdered sugar, cocoa powder, and vanilla and stir until smooth. Pour over the top graham cracker layer, spreading the glaze out to create a smooth top.
4 Tablespoons salted butter, 1/3 cup milk, 2 cups powdered sugar, 6 Tablespoons cocoa powder, 1 teaspoon vanilla extract
Transfer the eclair cake to the refrigerator and let it chill for 8 hours until set. Slice into squares and serve.