Peel, core, and slice apples. Mix sugar and cinnamon and toss with apples.
Melt butter in small (5"-6") cast iron skillet. Put milk in a 2-cup measuring cup, and pour half the butter into the milk. Whisk.
Add apples to remaining butter in skillet and toss to coat. Cook over medium heat until apples soften and begin to caramelize.
Add egg to milk and butter and whisk to combine. Add salt and flour and whisk until smooth. Off heat, pour batter over apples to cover as much as possible.
Put skillet in a 425 degree oven for about 10 minutes until pancake is set, slightly golden and puffy. Eat from skillet or invert onto a plate to serve. Can be eaten as is, or with syrup or a sprinkling of powdered sugar.