Remove about 18 walnut halves to split and garnish on cookies. In a food processor grind remaining walnuts.
In bowl, beat egg whites until soft peaks form; fold in sugar, cocoa and ground walnuts.
Drop by scant 1 tbsp, about 2 inches apart. onto parchment paper-line baking sheet. Top with a walnut piece. Bake in a 350 degree oven until set. 12 to 14 minutes. Transfer to cooking rack and let cool completely.