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You can crush them by pulsing them in a blender, or by placing them into a sandwich bag and smacking them with a rolling pin. Keep crushing the cookies until you get a coarse crumble, sort of like coarse sand or coffee grounds. You can use a fresh or frozen banana, but a frozen banana will give you a colder, thicker milkshake. You can also add some ice cubes, preferably make from frozen almond, cashew, or soy milk. The milk-based cubes will help thicken your milkshake without sacrificing flavor. You may have to pause the blender and scrape any unblended ingredients down the sides with a rubber spatula. Since you have already crushed the Oreo cookies beforehand, you just need to mix them in. You want to still see some specks of Oreo cookie. If this is too much for you, pour it into two smaller glasses and share it with a friend. If you don't feel like sharing, you can store the milkshake in the fridge for later. If the milkshake is not thick enough for you, put it in the freezer for about 10 minutes.
Crush the cookies into a fine crumble and set them aside. Put the milk, banana, and vanilla extract into a blender and blend until smooth. Add the crushed cookies into the blender, and pulse a few times until just combined. Pour the milkshake into a tall glass.