How to Make It
In a medium saucepan, combine all of the ingredients except the sparkling wine with 2 cups of water and bring to a boil. Simmer over moderately low heat until the plums are very soft and the syrup is deep purple, about 20 minutes.
Strain the syrup through a fine sieve set over a bowl, pressing on the solids. You should have 2 cups. Let the syrup cool completely, then refrigerate until chilled.
For each cocktail, pour 1 ounce of syrup into a flute and top with sparkling wine.