Combine 3 ounces gold rum, 1 ounce lime juice, 3/4 ounce demerara syrup, 1/4 ounce grenadine and 2 dashes of angostura bitters in a shaker with ice; shake well. Strain into a tall glass filled with ice. Garnish with a mint sprig and pineapple slice.
To make simple syrup, heat equal parts granulated sugar and water in a saucepan until the sugar dissolves; let cool. (You can use demerara sugar or honey instead of granulated sugar.)