In a saucepan, combine fresh blackberries, mint leaves, granulated sugar, and water. Stir well and place it over medium heat. Bring to a gentle boil and cook for about 5 minutes, until the blackberries soften and juices are released.

Once simmered, remove from heat and allow to cool for about 15 minutes. Strain the mixture into a bowl using a fine-mesh sieve, gently pressing the solids to extract maximum syrup.

Fill each glass with ice cubes. Pour 2 ounces of the blackberry-mint syrup into each glass over the ice.

Slowly pour chilled Prosecco into each glass, filling it to the brim. Stir gently to combine syrup and Prosecco.

Garnish each glass with additional mint leaves and whole blackberries before serving.