For an extra layer of indulgence, consider using a high-quality, dark chocolate syrup. To transform this milkshake into a spirited cocktail, add a shot of your favorite liqueur, such as vodka, rum, Kahlua, or Irish cream. Adjust the amount of chocolate syrup to your personal preference. For an even thicker milkshake, freeze the ice cream for about 15 minutes before blending. Prepare the milkshake just before serving to maintain its optimal texture and flavor.