Whisk together flour, sugar, baking powder, cinnamon, and salt in a large bowl. Set aside.: The dry mixture should smell faintly spiced and look homogenous with no streaks of flour remaining, which ensures even leavening and flavor; if you skip thorough whisking you risk pockets of baking powder or uneven cinnamon distribution, causing inconsistent rise and flavor.
In a separate bowl, gently beat the eggs and milk together. Pour this into the dry mixture and stir until just combined. Fold in diced apples.: You should hear a slight scrape of spoon against bowl as you fold, and the batter should be lumpy not smooth, indicating you have not overworked the gluten; overmixing yields tough fritters, while underfolding leaves floury pockets, so stop when the batter looks cohesive and the apple pieces are evenly suspended.
Heat the oil in a large pot to 375 degrees. Take care to monitor the temperature closely, if it's hotter, the outside of the fritters will cook too quickly, leaving the insides doughy. Carefully drop the batter by heaping spoonfuls into the hot oil. Do not overcrowd the pan, it's better to work in batches than to try and make these all at once. Depending on the thickness of your batter, you may want to flatten it out a little, so the fritter is not too thick. Cook to golden brown, approximately 2 minutes, then flip to cook the second side.: When the oil reaches temperature you will see a thin shimmer and a faint scent rather than smoke; if it is too cool the fritters will absorb oil and become greasy, too hot and the outside will darken before the inside cooks, so use a thermometer and adjust the burner to maintain 375 degrees.
Remove each fritter as it is golden brown all over. The second side will likely take less time than the first. Transfer to a plate lined with paper towels or a baking rack over a cookie sheet.: Each spoonful should sizzle immediately and form a round, bubbly edge, a sound that signals proper frying; if the batter sinks without sizzling the oil is too cool, and if it spits violently the oil is too hot, either issue will affect texture and color.
Prepare the glaze by whisking together powdered sugar, vanilla, and milk. Dip or drizzle glaze over each apple fritter then allow to cool slightly prior to serving.: The oil temperature will plunge if you add too many fritters, causing uneven cooking and greasy results; watch the surface for steady bubbling around fritters, which indicates consistent heat, and pause between batches to restore oil temperature.
Depending on the thickness of your batter, you may want to flatten it out a little, so the fritter is not too thick: A slightly flattened shape encourages even cooking all the way through, and you should see the edges begin to set and color while the center still jiggles slightly; if fritters are too tall the exterior will darken before the center cooks, leaving a doughy interior.
Cook to golden brown, approximately 2 minutes, then flip to cook the second side: Look for a warm honey color and listen for a steady, moderate sizzle; the second side typically takes less time because the fritter has already started to conduct heat inward, and flipping too soon or too often can deflate the fritter and interfere with browning.
Remove each fritter as it is golden brown all over: Transfer to a plate lined with paper towels or a baking rack over a cookie sheet so excess oil drains and the base stays crisp; stacking fritters while hot traps steam and softens the crust, so give them space to rest.
The second side will likely take less time than the first: Use visual clues more than a strict timer, noting the even golden color and a firmer feel when gently pressed; if you rely solely on time you may under or overcook depending on batch size and oil heat variability.
Transfer to a plate lined with paper towels or a baking rack over a cookie sheet: The fritters should release a faint toasty aroma and feel slightly crisp when tapped; placing them directly on paper will capture excess oil, whereas a rack preserves the bottom crispness, which is preferable to prevent sogginess.
Prepare the glaze by whisking together powdered sugar, vanilla, and milk: The glaze should be smooth and glossy, thick enough to coat a spoon yet thin enough to drizzle; lumps of sugar mean you need more whisking or a touch more milk, while an overly thin glaze will slide off instead of coating the fritters.
Dip or drizzle glaze over each apple fritter then allow to cool slightly prior to serving: As the glaze hits the warm fritter it will set with a satin sheen and release a sweet vanilla scent; wait a few minutes so the glaze firms up slightly and the fritter interior remains warm without being scalding, and avoid glazing immediately if they are still soaking oil as the glaze will not adhere well.