fresh rhubarb, chopped into 1/2-inch pieces
sugar (this makes for a relatively non-tart syrup. If you like it tart, use ¾ cup)
Prosecco (or another dry sparkling wine), chilled
To make the syrup, in a blender, combine the rhubarb and sugar. Pour in the boiling water, cover the blender, and blend until completely puréed. Strain through a fine mesh sieve and store the syrup in an airtight jar for up to a couple weeks in the refrigerator. Reserve the pulp for another use, like spreading on buttered toast or scones or stirring into some sautéed garlic and ginger to make a topping for pork tenderloin.
To make the cocktail, put 3/4 ounce of the chilled syrup in a cocktail coupe or Champagne flute, then top with the Prosecco. Adjust the amount of rhubarb syrup to taste.