Melt butter in an oven-safe skillet over medium heat; cook and stir apple slices, about 1/4 cup sugar, and cinnamon in butter until apples are tender, about 5 minutes.
Beat eggs, milk, flour, remaining 1/4 cup sugar, baking powder, vanilla extract, and salt in a large bowl until batter is smooth; pour batter evenly over apples.
Bake in the preheated oven until golden brown, about 10 minutes. Run a spatula around the edges of the pancake to loosen. Invert skillet over a large plate to serve.