This drink is a tweak of the standard Champagne Cocktail. The normal procedure is to soak a sugar cube in bitters, then pour champagne over it. This variation substitutes liqueur for the solid sugar, so the procedure is a little bit different, but the results are even more delicious.
1-2 tsp orange liqueur
1-2 dashes Angostura bitters
Add liqueur and bitters to a glass of sparkling wine and stir briefly to combine.
The nose carries light orange notes over the dry wine, which come together to produce something akin to peach, inflected with the cinnamon and allspice of the bitters. The sip recapitulates the nose, with elements of orange and bitter dancing around the sparkling wine without overwhelming it.
I really like the way this drink comes together. The liqueur/bitters combo balance each other out, keeping the drink in off-dry territory. Obviously you can change that balance by using more or less of each element. To me it comes off as a more austere Mimosa, with smooth orange flavors but retaining the crispness of dry sparkling wine due to the bitters. Basically, this is a nice way to tweak sparkling wine without losing its essential character.