For deeper flavor, add a pinch of salt to the syrup. Freeze glasses beforehand to keep the shake colder longer. If you do not have a torch, toast marshmallows briefly under a broiler. For a darker color, blend in a few whole blackberries directly into the shake. Best served immediately. Blackberry syrup can be refrigerated in an airtight container for up to 5 days.
Keyword berry dessert drink, blackberry milkshake, gourmet milkshake, toasted marshmallow shake