Notes: The first crêpes tend to be thicker and require slightly more batter to fill pan; as batter stands, it thins and spreads more readily. If making up to 3 days ahead, stack crêpes, wrap airtight, and chill; freeze to store longer. If you place a sheet of waxed paper on top of each crêpe as you stack them, the crêpes can be easily separated when cold. Otherwise, to avoid tearing, let the stack warm to room temperature or heat briefly in a microwave before separating.
1. 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.
2. 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.
3. 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.
4. 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.
Nutritional analysis per 7-in. crêpe.