1

Preheat the oven to 400°F. Line two 13 by 17-inch sheet pans with parchment paper.

2

In a stand mixer fitted with the whisk attachment, beat the egg whites and cream of tartar (if using) on medium-high speed for about 2 minutes, or until medium peaks form. Add 3/4 cup of the granulated sugar in a stream, and beat on high for 1 minute longer, or until stiff peaks form.

3

While the egg whites are beating, in a bowl large enough to fold together the finished cake batter, whisk the egg yolks with the rest of the granulated sugar for about 1 minute, until they are slightly pale.

4

With a rubber spatula, begin folding together the egg white and yolk mixtures. Place one-quarter of the mixture in a small bowl and fold in the melted butter.

5

Fold the butter mixture back into the egg mixture while sifting over the flour, about one-quarter at a time. Alternate between adding the flour-about one-fourth at a time-and folding until the flour is no longer visible.

6

With an offset spatula, spread the batter evenly in the prepared pans.

7

Use your thumb to make moats around the cakes to keep the batter from sticking to the sides of the pans. Bake for about 12 minutes, or until the tops of the cakes are firm to the touch. If the cakes seem to be cooking unevenly after 6 minutes, rotate the pans.

8

While the cakes are baking, sprinkle 2 sheets of parchment paper at least as large as the sheet pans with confectioners' sugar. Remove the cakes from the oven and flip out the cakes onto the sheets of prepared parchment paper by setting the cakes right next to the parchment.

9

Immediately peel away the cooked parchment paper from the bottoms of the cakes.

10

Let cool, then trim off any dried-out edges.