Pour enough vegetable oil into a heavy-bottomed pot or deep pan to reach a depth of 1½ inches. Place over medium-high heat and affix a thermometer to monitor temperature, heating the oil to 350°F (175°C). Adjust the heat as necessary to maintain consistent temperature.

Line a large rimmed baking sheet with paper towels. Position a wire cooling rack on top to drain the fritters post-frying.

In a large mixing bowl, combine the flour, sugar, baking powder, salt, and cinnamon. Whisk until evenly blended.

In a separate bowl, whisk together the milk, eggs, and vanilla extract until smooth. Pour the wet mixture into the dry ingredients and fold gently with a rubber spatula until nearly combined, leaving a few flour streaks.

Incorporate the diced apples into the batter, mixing just until the fruit is well coated and no dry flour remains.

Using a ¼-cup measure, portion the batter and carefully lower into the hot oil. Fry in batches of 4 to 6 fritters to avoid crowding.

Cook each fritter for approximately 2 minutes per side, or until deep golden brown throughout. Flip once halfway through.

Remove with a slotted spoon and place on the prepared wire rack to drain. Repeat until all batter has been used.

In a medium bowl, whisk together the powdered sugar, 3 tablespoons of milk, and the vanilla until smooth. Adjust consistency with additional milk if needed to achieve a pourable texture.

While the fritters are still warm, either dip each one directly into the glaze or drizzle the glaze over the tops with a spoon. Allow to sit on the wire rack for 10 minutes so the glaze can firm up.