Place eggs in a large bowl and whisk together with the sugar, milk, and vanilla until thoroughly combined. In a separate bowl, whisk together the salt, baking powder, cinnamon, and flour. Add the dry mixture to the wet a little at a time until fully mixed in.
Peel the apples cut them into fourths and remove the stem and seeds. Slice into three even slices. Toss with the cornstarch.
In a large skillet with deep sides, add enough oil to about 1/2 inch up the side of the pan. Heat over medium heat. Oil is ready when a tiny drop of batter placed into the oil will sink and immediately rise with bubbles all around.
Dip apple slices one by one into the batter, allowing excess to drip off. Carefully place into the oil and let fry, turning occasionally until golden brown on all sides. Let drain on a paper towel-lined sheet just to get off the excess oil. Place apple fries on a wired rack over a sheet try while frying the remaining apples.
Top with powdered sugar and serve with caramel sauce for dipping, optional.