To prepare cake, weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 1/2 teaspoon salt, baking soda, baking powder, and cocoa. Place 1/2 cup 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 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. Stir in red food coloring. 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 3 tablespoons butter and cream cheese in a large bowl, and beat with a mixer at medium-high speed until smooth.
Add powdered sugar, 1/4 teaspoon vanilla, and 1/8 teaspoon salt; beat until smooth. Add red food color paste to icing; stir.
Place frosting in a zip-top plastic bag; seal. Snip a 1/4-inch hole in 1 corner of bag. Pipe on top of cupcakes.
Stylish Swirls. Dot the thick frosting with a few drops of red food color paste, and stir gently to give it a swirled appearance. Carefully spoon into a zip-top bag, and pipe out.