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