Make the Lapsang tea simple syrup: Bring the water to a boil. Add the sugar and stir until it dissolves. Add the tea and turn off the heat. Let steep for 10 minutes. Strain out the tea leaves.
Make the cocktail: In a cocktail shaker, combine all the ingredients except for the ice and orange slice. Add the ice and shake vigorously for 20 seconds. Strain into a chilled rocks glass filled with ice. Garnish with an orange slice and serve.