Pour oil in a large cast iron skillet or heavy saucepan to a depth of at least 1 inch. Heat to 365° over medium-high heat. If you use a saucepan, you can use less oil to reach the 1-inch depth. You'll just fry fewer at a time.
Meanwhile, slice apples crosswise into 1/4-inch thick slices. Using a small circle cookie cutter remove center of apple so there’s about 3/8-inch apple remaining. You can also carve out the center with a knife or apple corer. You can toss the seeds and core but don’t throw away the apple in the centers! Chop them up in chicken salad or eat as a healthy snack (tomorrow after you’ve eaten the fried apples today!)
In a small bowl, combine 1/3 cup sugar and 1 teaspoon cinnamon to make Cinnamon-Sugar for coating apple rings.
In a medium bowl, whisk egg lightly; gradually add milk. Add flour; top with 2 tablespoons sugar, baking powder and salt. Whisk dry ingredients together lightly on top of milk, then dig in and whisk all together. I just saved you a bowl so you don't have to mix dry ingredients together separately.
Check your oil to see if it’s 365°. I like to use a digital thermometer. Using tongs, dip apple rings in batter, allowing excess to drip back in bowl. Fry 3 or four at a time 1 to 2 minutes or just until golden. Let drain on paper towels 1 minute; dredge them in then cinnamon sugar mixture while they are still warm and turn over with tongs to coat well. Eat warm and save any leftovers to warm them in the microwave the next day for breakfast.