Fill a small saucepan with water and bring water to a boil. Pour egg whites into a glass bowl that will fit just over the saucepan. Make sure the bottom of the bowl does not touch the boiling water. Whisk the egg whites continually for 2-3 minutes until they are foamy. Remove from stove. Whip egg whites until soft peaks form. Set aside.
In a separate bowl, mix together yogurt, orange juice, vanilla extract, SweetLeaf® Liquid Stevia Lemon Drop Sweet Drops™ or SweetLeaf® Liquid Stevia Valencia Orange Sweet Drops™, and half of the lemon zests and orange. Set aside.
In another small bowl, mix the rest of the zest with the ground shortbread cookie. Evenly divide the crumbled cookie into 4 serving dishes, saving 1/4 cup for the topping.
Fold the egg whites into the yogurt mixture. Pour the yogurt mousse into each serving dish. Sprinkle with the remaining crumbled cookie. Chill for an hour. Keep refrigerated until ready to serve.