Separate the egg whites and yolks into two clean bowls.
Add milk, vanilla, and lemon zest to the egg yolks and whisk briefly.
Sift in flour and baking powder. Whisk until smooth with no lumps. Set aside.
Add vinegar or lemon juice to the egg whites. Beat with a hand mixer on medium speed until frothy.
Gradually add sugar, then increase speed to medium-high and beat until stiff peaks form.
Gently fold one-third of the meringue into the yolk mixture.
Add the remaining meringue and fold gently until just combined. Do not overmix.
Transfer batter to a piping bag, cookie scoop, or use a large spoon.