Prepare pudding by mixing 2 packages of instant pudding with 2 cups cold milk in a large bowl. Whisk until smooth and set in the fridge for 10-15 minutes.
While pudding chills, make the chocolate glaze: In a saucepan, melt butter over low heat. Whisk in cocoa powder and powdered sugar until smooth. Gradually add 1/2 cup milk, stirring until mixture thickens and coats the back of a spoon (5 minutes). Remove from heat and let cool slightly.
In a separate bowl, layer half the graham crackers across the bottom of a 9×13 inch dish. Spread half the pudding over the crackers, then add half the whipped topping. Repeat with remaining layers, ending with a top layer of whipped topping.
Pour glaze evenly over the top layer, spreading to cover completely. Cover the dish and refrigerate for at least 4 hours or overnight.
Remove from fridge and slice into cubes or rectangles. Serve chilled.