1. Preheat oven to 350°.
2. To prepare cupcakes, lightly coat 24 muffin cups with cooking spray. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Weigh or lightly spoon potato starch into a dry measuring cup; level with a knife. Combine flour, potato starch, baking powder, and 1 teaspoon salt, stirring well with a whisk.
3. Place 1 3/4 cups sugar and 10 tablespoons butter in a large bowl; beat with a mixer at medium speed until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Combine milk and half-and-half. Add flour mixture and milk mixture alternately to butter mixture, beginning and ending with flour mixture. Beat just until combined. Fold in bourbon and 1 tablespoon vanilla. Divide batter evenly among prepared muffins cups; bake at 350° for 18 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 5 minutes. Remove cupcakes from pans; cool completely on wire racks.
4. To prepare frosting, combine 1 cup sugar and 1/4 water in a saucepan; bring to a boil. Cook 3 minutes, without stirring, or until candy thermometer registers 250°. Combine egg whites, cream of tartar, and dash of 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 white mixture, beating at high speed until stiff peaks form, about 3 minutes. Reduce mixer speed to low; continue beating until egg white mixture cools (about 12 minutes). Fold 1/4 cup unsweetened cocoa into cooled meringue mixture, and top each cupcake with about 2 tablespoons frosting. Sprinkle 1 tablespoon unsweetened cocoa evenly over cupcakes, if desired.