Combine water, sugar, and plums in a medium saucepan over medium-high heat; bring to a boil. Cook 5 minutes, stirring occasionally. Remove from heat; cool completely. Strain syrup through a sieve over a bowl; reserve plums. Stir eau-de-vie or brandy into syrup; chill. Discard plum skins; puree flesh until smooth. Chill puree. Spoon 2 teaspoons puree into each of 6 flutes; discard remaining puree. Divide syrup evenly among glasses. Top each serving with 1/3 cup prosecco; stir.