Preheat oven to 350 degrees F. Line an 8 x 11-inch pan with baking paper, sprayed with non-stick cooking spray.
Beat egg whites. In another bowl, beat egg yolks with the rest of the sugar substitute. Fold in rest of the ingredients by hand, except for cocoa and powdered sugar. Fold in beaten egg whites.
Spread three-quarters of the batter in the prepared pan. Mix cocoa and powdered sugar into the remaining batter and spread over what is already in the pan. Bake 15-20 minutes, until firm to the touch.
Remove from oven and, with a serrated knife, slice into 1/4-inch thick cookies. Occasionally dipping the knife into a glass of warm water will make the slicing easier and neater. Cover pan with a cookie sheet and quickly turn upside down. Carefully remove the baking paper and break the cookies apart.
Lower heat to 200 degrees F and put cookies back in the over for at least 90 minutes, until dry and golden at the edges, turning every 45 minutes.
Alternatively, turn off the heat and leave them in the oven overnight. Store in an airtight container. For those who like their cookies soft, just skip the drying process.