To make the sour mix: Boil water and pour into a small jar over the sugar. Stir until fully dissolved, then let cool to room temperature. Add the lemon juice and stir to combine.
To make the cocktail: In a pint-size Mason jar or a cocktail shaker halffilled with ice cubes, combine the rum, amaretto and 2 tablespoons of the sour mix. Cover and shake well.
Strain into a glass with 2 ice cubes, then top with bitters and serve.