Place the oven rack in the middle of the oven. Preheat oven to 450 degrees F.
Melt the butter in 10 to 11-inch ovenproof skillet over moderate heat, then transfer 2 tablespoons of the butter to a blender. Add the apple wedges to the skillet and cook, turning over once, until beginning to soften, 3 to 5 minutes.
While the apple is cooking, add the milk, flour, eggs, sugar, vanilla and salt to the butter in the blender and blend until smooth.
Pour the batter over the apple in the skillet and transfer the skillet to the oven. Bake until the pancake is puffed and golden, about 15 minutes. The pancake will puff up a lot in the oven and sink down when removed from the heat. Dust the pancake with powdered sugar, cut into slices and serve immediately.
Notes
*Recommended sweet apples: Gala or Golden Delicious.