Melt chocolate in top of a double boiler or in a metal bowl set over (but not touching) a saucepan of simmering water.

In a mixing bowl, using high speed of electric mixer, beat egg whites and cream of tartar until soft peaks form. Add vanilla, then add sugar a few teaspoons at a time, beating all the while until egg whites are stiff but not dry. Fold the warm melted chocolate into the egg white mixture.

Using a tablespoon measure, drop 12 neat rounds of egg/chocolate onto cookie sheet, spacing about 2 inches apart. Bake on center rack for 8-10 minutes – tops will appear set and slightly cracked.

Slide parchment or mat off the tray and allow cookies to cool. Slide a metal spatula underneath the cookies and scoop them off the sheet when cool.