Put oven rack in middle position and preheat oven to 450 degrees F (230 degrees C).
Melt Unsalted Butter (1/4 cup) in skillet over moderate heat, then transfer 2 tablespoons to a blender or food processor. Add Golden Delicious Apple (1) to skillet and cook, turning over once, until beginning to soften, 3-5 minutes.
While apple is cooking, add Whole Milk (1/2 cup), All-Purpose Flour (1/2 cup), Large Egg (4), Granulated Sugar (3 tablespoon), Vanilla Extract (1/2 teaspoon), and Salt (1/4 teaspoon) to butter in blender and blend until smooth.
Pour batter over apple and transfer skillet to oven. Bake until pancake is puffed and golden, about 15 minutes. Dust with Powdered Confectioners Sugar (to taste) and serve immediately.