1. Heat oven to 350 degrees F. Line a standard cupcake pan with 12 paper or foil liners.Cupcakes:
2. Whisk flour, baking powder and salt in small bowl.
3. In large bowl, beat butter with an electric mixer. Gradually add sugar; beat until lightened. Slowly beat in egg whites. Add flour mixture alternating with milk, beginning and ending with flour. Beat in vanilla extract.
4. Place 1 cup batter in small bowl. Stir in melted chocolate. Fill six prepared liners with white batter, six with chocolate. Bake at 350 degrees F for 20 to 25 minutes, or until crowned and lightly colored. Remove from pan; cool on rack.
5. Remove cupcake liners and slice in half (separate tops from bottoms).Frosting:
6. In medium-size bowl, with an electric mixer on low speed, beat together sugar, butter, shortening, milk and vanilla extract. Remove 3/4 cup frosting to a small bowl; stir in melted chocolate. Add additional milk, if necessary.
7. Spread white frosting on white cupcake bottoms, and chocolate frosting on chocolate bottoms. Top white bottoms with chocolate tops and finish with additional white frosting. Repeat with white cupcake tops and chocolate frosting.