Put oven rack in middle position and preheat oven to 450°F.
Melt butter in a 10 inch oven-safe skillet over moderate heat, then transfer 2 tablespoons to a blender or food processor.
Add apple wedges to skillet and cook, turning over once, until beginning to soften, 3 to 5 minutes.
While apple is cooking, add milk, flour, eggs, granulated sugar, vanilla, and salt to butter in blender and blend until smooth.
Pour batter over apple (while pan is still on/hot) and transfer skillet to oven. Bake until pancake is puffed and golden, about 15 minutes.
Optional - Dust with confectioners sugar and serve immediately.
