In a bowl with mixer on high speed, beat egg whites and cream of tartar until they form a thick foam. Continue to beat, gradually adding sugar until whites hold stiff moist peaks.
In a blender, whirl egg yolks, milk, flour, and baking powder until smooth; scrape container as needed. Pour into whipped whites; fold gently until mixture is thoroughly blended.
Use a crêpe pan or regular or nonstick frying pan with a bottom that measures 7 to 8 inches across. Set pan on medium heat; when hot, brush bottom with butter (as needed or for flavor, in a nonstick pan). Pour 1/2 to 2/3 cup batter into pan all at once, and with the back of a spoon spread to cover pan bottom evenly. Cook until crêpe is golden brown on both sides, turning once with a wide spatula, 1 1/2 to 2 1/2 minutes total.
Invert crêpe onto a flat plate; cover with a sheet of plastic wrap. Repeat to cook and stack remaining crêpes (it gets thinner as it stands; stir often), tilting pan to coat bottom evenly.