Allow butter and egg whites to stand at room temperature for 30 minutes. Meanwhile, line twenty-four 2-1/2-inch muffin cups with paper bake cups. In a medium bowl stir together flour, baking powder, salt, and baking soda. Set aside.
Preheat oven to 350 degrees F. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar and vanilla; beat until combined. Add egg whites, one at a time, beating well after each addition. Alternately add flour mixture and buttermilk to butter mixture, beating on low speed after each addition just until combined. (Batter may appear curdled.) Fold in chocolate pieces.
Spoon batter into prepared muffin cups, filling each about two-thirds full. Use the back of a spoon to smooth out batter in cups.
Bake for 15 to 18 minutes or until a wooden toothpick inserted in centers comes out clean. Cool cupcakes in muffin cups on wire racks for 10 minutes. Remove cupcakes from muffin cups. Cool completely on wire racks.
Just before serving, spoon Ricotta Frosting into a pastry bag fitted with a large round tip. Insert tip into the top of each cupcake; squeeze some of the frosting into the center of each cupcake. Generously pipe remaining frosting onto tops of cupcakes. Sprinkle with pistachio nuts and grated chocolate. Store, covered, in the refrigerator.