Preheat oven to 350 degrees F. Line 24 standard cupcake cups with paper liners. Set aside.
In a medium grease free bowl, beat egg whites to soft peaks with an electric mixer. Slowly add 1/3 cup of sugar and beat to stiff peaks. Set aside.
In a large bowl, sift together the cake flour, sugars, baking powder and salt. Add remaining ingredients and beat with same mixer (no need to wash beaters) until combined (about 1 minute). Fold egg whites into the batter being careful not to deflate the egg whites. Spoon mixture, a little over half full, into each cupcake cup liner. Bake for approximately 18 minutes or until skewer inserted comes out clean. Let cupcakes cool in pan for about 5 minutes then remove to a wire rack to cool completely.
Make Buttercream: Place water and sugars in a small saucepan. Stir to combine. Bring to a boil over medium heat and continue to cook for approximately 5 minutes. Sugar will be dissolved. Whip egg whites in a stand mixer (make sure bowl is grease free) to stiff peaks. Slowly add hot sugar syrup to egg whites and continue beating on high speed until bowl is cool to the touch about 10 to 15 minutes. Add butter a piece at a time and mix completely before adding the next piece of butter. Repeat until all butter has been used. Add confectioners' sugar. Mix to combine. Add orange liqueur and 2 teaspoons of zest. Mix to combine. Frosting will be smooth and creamy.
Generously frost cupcakes with a piping bag or spatula and top with coarsely grated orange zest.
Recipe courtesy of Kelly Madey, Quakertown, PA