Position oven racks in the upper and lower third of the oven and heat the oven to 350 F. Lightly grease two baking sheets or line them with parchment.

Melt the chocolate and set aside. Using a hand or stand mixer, beat the egg whites with the cream of tartar until soft peaks form. With the beaters running, gradually add the vanilla and sugar until the egg whites hold stiff peaks but dont look dry. Pour the nuts and melted chocolate over the whipped whites. Gently fold the mixture with a large rubber spatula, trying not to deflate the egg whites, until the color is just uniform. Immediately drop level measuring teaspoons of the batter onto the baking sheets, leaving at least 1 inch between the cookies.

Bake until the cookies are shiny and cracked, 10 to 12 min.; they should be firm on the outside but still gooey inside when you press them. Slide the parchment liners onto racks or use a metal spatula to transfer the cookies to racks and let cool completely. The cookies are best eaten on the day theyre baked but will last two to three days if stored in a n airtight container.