Fill a cocktail mixing glass with ice. Pour in bourbon, syrup* and bitters. Stir until cold.
Strain cocktail into a rocks glass over a large ice cube. Garnish with lemon peel.
Add frozen peaches, sugar, and water to a small saucepan and bring to a simmer over medium-low heat. Simmer for ten minutes, then remove from heat and let sit for 30 minutes to cool. Strain through a fine mesh strainer into a bottle. Syrup can be stored in an airtight container in the refrigerator for up to 1 week.