Add the chopped plums, water, sugar, orange zest, and cinnamon stick to a small saucepan.

Bring the mixture to a gentle simmer over medium heat.

Stir until the sugar fully dissolves.

Simmer for about 10 minutes until the plums soften and the liquid turns deep purple.

Remove the saucepan from the heat and let the mixture cool slightly.

Strain the syrup through a fine mesh sieve into a clean jar.

Discard the solids and set the syrup aside to cool completely.

Add the bourbon to a mixing glass.

Pour in the sugar plum syrup.

Add two dashes of Angostura bitters.

Stir the mixture until chilled.

Place one large clear ice cube into a lowball glass.

Strain the cocktail over the ice cube.

Curl a thin strip of orange peel and place it inside the glass on the right side.

Add a small slice of plum as an optional garnish.

Serve immediately.