*Rich Brown Sugar Syrup: Add 1 cup dark brown sugar (packed) to ½ cup water. Add the brown sugar and water to a small saucepan. Heat gently over medium-low, stirring until the sugar fully dissolves. Once dissolved, remove from heat—do not boil. Cool completely, then transfer to a clean bottle or jar.
Add bourbon, brown sugar syrup, bitters, and a small pinch of sea salt to a mixing glass.
Stir until well chilled—about 20 seconds.
Strain into a rocks glass over one large cube.
Express the citrus peel over the drink, then drop it in.