Step 1Melt the butter and let it cool slightly.
Step 2Meanwhile, stir together the flour, baking powder, sugar and salt in a mixing bowl. Whisk in the milk, half-and-half and zest. The mixture will be very liquid; don't worry.
Step 3Whisk in the egg yolks (this will thicken the batter slightly). In a separate bowl, beat the egg whites until soft peaks form and stir them gently into the batter. (You don't need to fold them; the batter is not that delicate.) This will thicken it to about the consistency of a good homemade eggnog. Whisk in the melted butter. (The recipe can be made ahead to this point and refrigerated, tightly covered, overnight.)
Step 4Heat a nonstick skillet over medium-high heat until drops of water skitter across the surface. Slowly pour one-half cup of the batter in the center, forming as much of a circle as you can (using a ladle or measuring cup with a lip makes this easier).
Step 5Cook until the bottom of the pancake has lightly browned and the top begins to look slightly dry, about 3 minutes. Flip the pancake and cook until it feels somewhat firm when pressed lightly in the center, 2 more minutes.
Step 6Remove from the pan and keep warm in the oven. Serve 2 pancakes per person.