To prepare cake, weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 1/2 teaspoon salt, baking soda, and baking powder. Stir in 1/2 cup cocoa. Place butter and 1 1/4 cups sugar in a large bowl; beat with a mixer at high speed until well blended. Add egg yolks, 1 at a time, beating well after each addition. Stir in 1/2 teaspoon vanilla extract. Reduce mixer speed to low. Add the flour mixture and buttermilk alternately to butter mixture, beginning and ending with flour mixture, and beat just until combined. Melt unsweetened chocolate, and stir into batter. Using clean, dry beaters, beat 3 egg whites and 1/4 teaspoon cream of tartar at high speed until stiff peaks form. Fold one-third of egg whites into batter. Gently fold in the remaining egg whites.
Line 24 muffin cups with cupcake liners; coat with baking spray. Spoon batter into cups. Bake at 350° for 23 minutes or until a wooden pick inserted into centers comes out with moist crumbs clinging. Cool in pans 10 minutes. Remove from pans; cool completely on wire racks.
To prepare frosting, place 1 1/3 cups sugar, water, and vanilla bean in a saucepan; bring to a boil. Without stirring, cook 3 minutes or until a candy thermometer registers 250°; discard vanilla bean. Combine 3 egg whites, 1/4 teaspoon cream of tartar, and 1/8 teaspoon salt in a large bowl; using clean, dry beaters, beat with a mixer at high speed until foamy. Pour hot sugar syrup in a thin stream over egg whites, beating at high speed until stiff peaks form. Reduce mixer speed to low; continue beating until mixture cools (12 minutes). Fold in 1/4 cup cocoa.
Top cupcakes with frosting. Shave bittersweet chocolate over frosted cupcakes.
Proper Chocolate Shavings: Use a block or bar of good chocolate for your garnish of chocolate curls. If you rub your thumb over the surface two or three times and then shave, the curls will be less brittle and less likely to crack and break.