Splash a puddle of Angostura® Bitters into a saucer and place the Sugar cube in it to soak. This will take a minute or two. Stir the Champagne (or sparkling wine), the Vodka and the Blue Curacao together in a container and put into the fridge to chill. Place the soaked Sugar cube in the bottom of the Champagne glass, pour in the Mixture and add drop in the cocktail Onion, which should be loose, not on a cocktail stick.