Preheat oven to 350ºF. Line cupcake pans with paper liners.
Combine flour, sugar, baking powder, salt, shortening, milk and vanilla in a large mixing bowl. Mix at low speed for 2 minutes. Scrape bowl. Add egg whites and additional whites or yolks, and mix at high speed until fluffy and smooth, approximately 2 minutes.
Fill liners 1/2 to 2/3 full of batter. Do not overfill. Bake 20-25 minutes or until toothpick inserted in center comes out clean. Let cool 10 minutes in pans then remove from pan, and place on wire racks to cool completely
To decorate the cupcakes: Center a cupcake on a turntable.
Take the green buttercream bag in hand and, while turning the turntable, apply steady pressure to pipe a border around the edge of the cupcake.