Grate the apple into large pieces, also catching the juice. Put the apple pieces and the juice in a tall mixing vessel, add the milk. Beat with the mixer attachments of the hand mixer until foamy, add cinnamon and, depending on your taste, liquid sweetener. Pour the shake into a large glass and enjoy immediately.
The entire shake has about 140 kcal using 0.1% milk. If you want to prepare the normal variant, simply replace the milk with whole milk and the sweetener with sugar.