For the Fritters:
Fill a large pot or Dutch oven with vegetable oil 1½ inches deep. Place over medium high heat with a candy or deep fry thermometer attached to the side. Heat the oil to 350°F. Adjust temperature if needed.
Line a large rimmed baking sheet with paper towels and set a wire rack on top of the towels.
While the oil is heating, make the batter. In a large bowl whisk together the flour sugar, baking powder, salt, and cinnamon.
In a medium bowl, whisk together the milk, eggs, and vanilla until well combined. Pour into the dry mixture. Use a spatula to fold the mixture together until just a few streaks of flour remain.
Add the apples and mix together so all the apples are coated and the flour is incorporated.
Once the oil is to temperature, scoop by the batter by about ¼ cup per fritter into the hot oil. I suggest frying about 4 to 6 fritters at a time.
Fry until golden brown on the bottom, about 2 minutes. Flip and continue frying until golden brown on the other side, about 2 more minutes.
Use a slotted spoon to transfer fritters to the wire rack and continue frying until better is all used up.
For the Glaze:
In a medium bowl, whisk together the powdered sugar, 3 tablespoons milk, and vanilla. You want a thin glaze, so add more milk if needed. Either dip the fritters in the glaze and allow to set on a wire rack or use a spoon to drizzle the glaze over the hot fritters. Let the fritter sit for 10 minutes to allow the glaze to set before serving.