The old "saw off slices of ice cream while it's still in the tub" is one of the oldest ice cream hacks in the book (the "book" being "Pinterest). It's great for making big, beautiful ice cream sandwiches, but for sandwiches made with smaller cookies, a plastic cup is the way to go.
First, you have to figure out what size cup you need. An Oreo is 1.75 inches in diameter, so you're going to want a cup that is around that size (no need to be exact). Since most cups get gradually wider towards the top, make sure the top diameter isn't too much bigger than your cookie of choice. (Restaurant supply stores have a lot of disposable cup options.)
Once you have selected a cup, scoop the ice cream in and pack it down as tightly as you can. Freeze for a couple hours and, once the ice cream has hardened, slice the ice cream (still in the cup) into perfect little patties. Peel off the pieces of cup and place your round in between two of your favourite cookies.
We should note that this hack is entirely cosmetic and that beautiful ice cream sandwiches don't taste any better than dipping cookies directly into a bucket of ice cream, chip and dip style (or so I've heard). But if you need your frozen treats to be Instagram-worthy, this little trick will help you make gorgeous sandwiches every time.
The Ice Cream Sandwich Trick + S'mores Ice Cream Sandwich Recipe [The Pinning Mama]