Make the sugar plum syrup: Simmer all ingredients over low heat for 10 minutes, stirring occasionally.Strain, cool, and store in the fridge for up to two weeks.
Add bourbon, syrup, and bitters to a mixing glass with ice.
Stir until well chilled—about 20 seconds.
Strain into a rocks glass over one large cube.
Garnish with a plum slice, rosemary sprig, or both.