For the Black Pepper Date Simple Syrup, bring all ingredients, except cornstarch, to simmer in medium saucepan, stirring to dissolve sugar. Strain mixture through a fine mesh sieve, being careful not to press too much of the pulp solids through the strainer. Place cornstarch in small bowl. Add 2 tablespoons of the strained date mixture with the cornstarch; mix well.
Return remaining strained date mixture back to the saucepan. Stir in cornstarch mixture with whisk until well blended. Bring to simmer. Cook and stir 1 minute. Remove from heat. Cool slightly. Store syrup in tightly covered, clean container in refrigerator until ready to use. (Makes 1 1/2 cups syrup. Enough for 12 Black Pepper Date Cocktails.)
For 1 Black Pepper Date Cocktail, place ice in cocktail shaker. Add Black Pepper Date Simple Syrup, rum, elderflower liqueur and lime juice; shake until well mixed and chilled. Strain into beverage glass. Garnish with a sprig of fresh rosemary, if desired. Serve immediately.