Let eggs stand at room temperature for 30 minutes. In a large bowl, cream the butter, shortening and sugar until light and fluffy. Add egg yolks, one at a time, beating well after each addition. Stir in extracts. Combine the flour, cornstarch, baking soda and salt; add to creamed mixture alternately with buttermilk, beating well after each addition.
In a small bowl, beat egg whites on high speed until stiff peaks form. Fold into batter. Stir in coconut and pecans.
Fill paper-lined muffin cups three-fourths full. Bake for 20–25 minutes or until toothpick inserted near the center comes out clean. Cool 10 minutes; remove from pans to wire racks to cool completely.
In a large bowl, combine the frosting ingredients until smooth; frost cupcakes. Store in refrigerator.