Add the bitters to the cube and place in the bottom of a Champagne flute. Add a splash of Campari, and then fill it up with Champagne.
Note that Angostura Bitters are a very specific type of bitters with a very unique flavor. You're only using a drop here, remember, and it makes a difference! I don't know any way of "substituting" for the Angostura Bitters. Luckily, most liquor stores carry this. Look for the small bottle with the paper wrap around it, as shown in the above photo.
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Our Sangria Recipes include a section on sparkling sangria recipes. These are Champagne Cocktails as well!