Heat oven to 425 degrees. Put butter in a 10- to 12-inch oven-proof frying pan or 10- to 12-inch shallow casserole. Set pan in hot oven until butter is lightly browned, 7 to 8 minutes. Tilt the pan to be sure bottom and sides are well coated.
While butter melts, place eggs, milk and flour in a blender or food processor. Whirl until very smooth. (Or, with an electric mixer or whisk, beat eggs and flour to mix well; add milk and beat until very smooth.)
Pour batter into hot pan in oven. Bake until pancake is a rich brown and sides have puffed, 25 to 30 minutes. Sift confectioners' sugar liberally over pancake, then cut into wedges. Serve at once with lemon wedges to squeeze over individual portions or with maple syrup to taste. Makes 6 servings.