Prep for frying: Pour 2–3 inches of vegetable oil into a deep cast iron skillet or heavy pot. Slowly heat over medium until it reaches 350–375°F. Line a plate with paper towels. Mix the cinnamon sugar in a shallow bowl.
2 cups vegetable oil, ¼ cup sugar, 1 teaspoon cinnamon
Make batter: In a large bowl, whisk together flour, sugar, cinnamon, baking powder, and salt. Add milk, maple syrup, egg, and oil. Whisk until smooth and thick.
1 ½ cups flour, 3 tablespoons sugar, ½ teaspoon cinnamon, 1 teaspoon baking powder, ⅛ teaspoon salt, 1 cup milk, 1 tablespoon maple syrup, 1 egg, 1 tablespoon oil
Prep apples: Peel, core, and slice 4 medium apples into ½-inch rings. Pat dry thoroughly with paper towels.If you struggle with batter grip, you can lightly dust the apple slices with cornstarch.
Fry: Working one at a time, dip each apple ring into the batter and gently lower into the hot oil. Fry 1–2 minutes per side until golden and puffy.
Finish: Drain briefly on paper towels, then immediately toss in cinnamon sugar. Set aside on a plate. Continue until all apple rings are fried.