
Add 1 tablespoon rosemary needles to small bowl. Muddle rosemary needles with muddler or back of wooden spoon to release oils.
Place small saucepan on stovetop over high heat. Add muddled rosemary needles, 1 ½ cups frozen raspberries, juice of 1 lemon, 5 tablespoons sugar, ¾ cup water, and 1 tablespoon cornstarch to saucepan. Stir to incorporate, then bring mixture to rolling boil.
When mixture begins to boil, immediately reduce heat under saucepan to low.
Use potato masher to mash raspberries completely, then stir mixture and simmer 10 minutes.
After 10 minutes, remove saucepan from heat and set aside. Let mixture cool completely.
Once cooled, pour mixture through fine-mesh strainer or nut-milk bag and into airtight container. Use immediately or refrigerate until ready to use.
Add 1 ½ ounces vodka (or gin), ½ ounce St. Germaine elderflower liqueur, 1 ½ ounces raspberry-rosemary syrup, and ice to cocktail shaker. Shake vigorously.
Strain cocktail into glass, with or without ice. Add 1 tablespoon soda water and stir to incorporate into drink. Top cocktail with 1 tablespoon champagnethen garnish with rosemary sprigs and raspberries if desired. Serve immediately.