One of the byproducts of producing sugar is molasses. This molasses, once fermented, is distilled into rum, which is why some of the early English-style rums were very molasses-heavy. Gosling's comes from that blackstrap molasses tradition and actually has the trademark for the Dark 'n' Stormy. But for this drink, you can use any sort of dark, demerara-style rum.
In a cocktail shaker, add dark rum, lime juice, and ginger beer. Add ice, seal, and shake. Using a Hawthorne strainer, strain into a tall glass with ice. Top with sparkling water, if desired, and serve.