Add ingredients to a cocktail shaker and shake over ice. Strain into a double rocks glass over one large ice cube. Garnish with a pineapple wedge and two maraschino cherries skewered on a cocktail pick. Tuck two pineapple leaves into the glass, if you have them.
Combine brown sugar and water in a small saucepan over medium heat. Stir until sugar is fully dissolved. Turn off the heat and let cool to room temperature. Store in an airtight container in the fridge for up to 3 weeks.