In a small saucepan combine blackberries, sugar, and water. Mash blackberries with a muddler, potato masher, or fork. Bring to a boil and then lower heat to a simmer for 3 minutes. Remove from heat.
Use a mesh strainer to strain solids from simple syrup. Set strained syrup aside to cool slightly. Blackberry Simple Syrup makes enough for 6 martinis.
In a cocktail shaker place vodka, 1 ounce blackberry simple syrup, elderflower liqueur, and lemon juice. DO NOT POUR IN PROSECCO YET. Fill with ice, shake, then strain into a martini glass. Top Blackberry Martini with Prosecco and serve.
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