Heat deep fryer or heat vegetable oil (2 inches deep) in a medium pot to 375 degrees.
Mix the cornstarch, salt, and flour in a bowl and add lightly beaten egg, vanilla, seltzer, and cinnamon. Mix with a fork, don't over beat. If batter is too thin add a bit more flour.
Dip apple slices in batter and drop into hot oil careful not to overcrowd. Let fry until light golden brown, moving the apples around in the oil so they brown evenly. Remove and let drain on paper towels. Serve warm as is or amp it up with whipped cream or vanilla ice cream! Enjoy!