Whisk the flour, confectioners' sugar, baking powder and salt in a large bowl. Whisk the milk, egg yolk, melted butter and vanilla in a medium bowl until combined. In a separate large bowl, beat the 3 egg whites with a mixer on medium-high speed until stiff peaks form, about 2 minutes.
Stir the milk mixture into the flour mixture with a rubber spatula until just combined (it's OK if there are a few lumps). Stir in one-third of the beaten egg whites, then gently fold in the rest until just combined (do not overmix).
Heat a griddle or large nonstick skillet over medium-low heat; lightly brush with butter. Pour 1/4 cup batter onto the griddle for each pancake; cook until bubbles form on top and the bottom is lightly browned, about 3 minutes. (If the pancakes are browning too quickly, reduce the heat to low.) Flip and continue cooking until golden on the other side, about 1 more minute. Transfer to a plate. Repeat with the remaining batter, brushing the pan with more butter as necessary. Serve with butter and syrup.