Preheat oven to 325 degrees. In a food processor, or in a plastic bag with a rolling pin, finely crush graham crackers.
In a medium bowl, combine graham cracker crumbs, butter, and sugar. Pour into a 9-inch pie plate, using the bottom of a measuring cup to press the crust evenly and firmly into the bottom and up the sides of the pie plate.
Bake until golden brown and fragrant, 15 to 20 minutes. Remove from oven and cool completely, at least 1 hour.
To make the filling:
In a large bowl combine sugar, cream cheese, and vanilla extract. Stir until evenly blended. Add cocoa powder and milk in alternating batches, a few tablespoons at a time, until evenly incorporated.
Gently fold in whipped topping until well-combined. Pour into cooled crust and smooth the top with a spatula. Freeze at least 4 hours or until serving (up to 1 month in advance).
Thaw for 5 to 10 minutes before slicing. Drizzle individual pieces with chocolate syrup if desired.