To make the simple syrup, add ingredients to a small saucepan. Bring the mixture to a boil over medium-high heat. Once the sugar is dissolved, remove from heat, cover, and steep for 20 minutes.
Cool syrup completely before using in the cocktail.
To make the cocktail, peel and slice plums. Add plum slices to the bottom of a cocktail shaker. Add simple syrup and lemon juice, muddling for 30 seconds.
Add vodka and fill the shaker with ice. Shake well.
Strain the cocktail evenly between two glasses that have been filled with ice.
Top off each glass with prosecco until full.
Garnish with extra plum slices and cinnamon sticks. Serve immediately.