Position a rack in the center of the oven and preheat to 325* F.
Put the egg whites and lemon juice in the bowl of a stand mixer fitted with the whip attachment. (Be sure bowl is immaculately clean, as any grease will prevent whites from whipping to stiff peaks) Whip whites on low speed for 2 minutes and then increase speed to high whip until stiff peaks form, about 5 minutes.
Sift the sugar and cocoa powder a bowl and then fold into the batter with a rubber spatula until batter is smooth and shiny. Fold in the walnuts, until they are well coated with batter.
Line cookie sheets with parchment paper or use silpat, silicone baking sheets. Drop heaping tablespoons of batter 1/2 " apart , being sure to include about the same number of walnuts (3 or 4) in each cluster.
Bake until outside has crisped and the bottom starts to pull away from the parchment paper, 15 to 20 minutes.
Remove the cookie sheet from the oven. As soon as the clusters can be moved, use a spatula to transfer them to a rake and let them cool.
Store in an airtight container for up to one week. They won't last more than a day though!