In a large bowl, mix the milk, pudding mix, and whipped topping together until fully combined. Set aside;
In the bottom of a 13x9 inch baking pan, arrange a single layer of graham cracker squares (you can break the squares into smaller pieces to get into the corners of the pan);
Spread half of the pudding mixture over top of the graham crackers;
Top with another layer of graham crackers;
Spread the remainder of the pudding mixture overtop;
Top with one final layer of graham crackers;
Spread the prepared frosting overtop of the entire cake, and use an offset spatula to spread it right into the corners (chill the cake for 30-minutes prior to help making spreading easier);
Cover the pan with plastic wrap and chill in the fridge for at least 4 hours before serving;