Add the Demerara Sugar Syrup, lime juice, light rum, aged rum and mint leaves to a mixing glass. Heavily muddle the mint leaves, add large ice and shake vigorously in a cocktail shaker. Taste for balance, double-strain over crushed ice in a rocks glass and garnish with a mint sprig and lime wedge.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.