In a large bowl, whisk together flour, sugar, baking powder, salt and cinnamon. In a medium bowl, combine egg, milk and vanilla. Add wet ingredients into the dry and stir just to combine. Let batter rest in fridge for 30 minutes.
Make cinnamon sugar; combine cinnamon and sugar in a large dish and set aside.
In a deep fryer, or wide, heavy bottomed pot no more than half full, heat oil to 375°F (190°C). Peel, core and slice apples into 1/2-inch (1.25 cm) thick rings. Toss slices with lemon juice to stop them from browning.
Dredge apple rings lightly in flour, shaking off excess. Dip into batter to evenly coat. Fry in small batches, turning once. Fritters are ready when batter is golden brown and apple is tender, about 3-4 minutes. Drain on a paper towel lined tray. While fritters are still warm, toss lightly in cinnamon sugar.