Whip cream until slightly thickened, then beat in cocoa, then sugar. Finished cream should be thick but not stiff. Texture should be about like thick ketchup. Use a whisk to finish adding sugar if it looks like beating will stiffen the cream too much.
Pour some of the cream into the pan and add a layer of graham crackers, placing them flat. Cover with cream. Add another layer of crackers and repeat until everything is used up. The top layer should be cream. Freeze the dessert until it is solid, then remove from freezer and allow to thaw completely before serving. Garnish with shaved chocolate if desired.