Add bourbon, hot chocolate syrup and chocolate bitters to a mixing glass with ice. Stir for about 30 seconds to chill and dilute.
Strain into a rocks glass over a large ice cube.
Garnish with a skewer of marshmallows. Toast marshmallows using a kitchen torch.
Hot Chocolate Syrup
Add all ingredients to a small saucepan. Heat over medium heat until sugar and mix are dissolved, stirring frequently. Remove from heat and cool before using.