How to Make It
Melt butter in a small microwavable bowl; set aside to cool slightly.
Whisk egg whites in a medium bowl until soft peaks form; set aside.
Whisk together flour, sugar, baking powder, and salt in a medium bowl. Create a well in the center of dry ingredients. Whisk milk, melted butter, and egg yolks into center of the dry ingredients; stir to incorporate dry ingredients. Place 1/3 of the egg whites onto center of batter. Use a spatula to cut down the center of egg white, and fold gently into the batter, scooping along the bottom of bowl, and folding over remaining batter. Turn bowl clockwise after each fold. Repeat with remaining egg white until all is incorporated, being careful not to overmix.
Heat a large nonstick skillet or griddle over medium. Add enough butter to coat bottom of skillet, and pour about 1/3 cup batter in hot butter. Cook, undisturbed, until bubbles have formed over surface and edges are getting golden brown, about 3 minutes. Flip, and cook until cooked through, 1 to 2 minutes. Repeat with remaining butter and batter.