In a small saucepan over medium-high heat, bring 1/2 cup sugar and 1 cup water to a boil. Add 1/2 cup fresh mint leaves. Reduce heat to maintain a simmer and cook until slightly thickened, about 10 minutes. Transfer syrup to a small metal bowl and let sit until cool, about 10 minutes. Use warm, let cool to room temperature, or store, covered and chilled, up to six months.